Member for Gas Sensor, Having a Metal Oxide Semiconductor Tube Wall with Micropores and Macropores, Gas Sensor, and Method for Manufacturing Same

ABSTRACT

Disclosed are a gas sensor member, a gas sensor using the same, and manufacturing methods thereof, and specifically, a gas sensor member using a one-dimensional porous metal oxide nanotube composite material having a double average pore distribution in which mesopores (0.1 nm to 50 nm) and macropores (50 nm to 300 nm) are simultaneously formed on the surface of a nanotube through decomposition of a spherical polymer sacrificial template and continuous crystallization and diffusion of a metal oxide and a nanoparticle catalyst embedded in an apoferritin is uniformly loaded in the inside and on the outer wall and inner wall of a one-dimensional metal oxide nanotube through a high-temperature heat treatment, a gas sensor using the same, and manufacturing methods thereof are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 15/111,173, having an effective filing date of Dec. 15, 2015and a 35 U.S.C. § 371 completion date of Jul. 12, 2016, which is a U.S.national phase application under 35 U.S.C. § 371 of internationalapplication no. PCT/KR2015/013707 having an international filing date ofDec. 15, 2015, which claims priority to Republic of Korea patentapplication no. 10-2015-0148273 filed on Oct. 23, 2015, Republic ofKorea patent application no. 10-2015-0034024 filed on Mar. 11, 2015, andRepublic of Korea patent application no. 10-2014-0186846 filed on Dec.23, 2014, all of which are incorporated by reference herein in theirentirety.

TECHNICAL FIELD

Embodiments of the inventive concepts described herein relate to anoptimal sensing material structure in which the diffusion and reactionof a gas take place rapidly and a manufacturing method thereof. Moreparticularly, embodiments of the inventive concepts relate to a metaloxide semiconducting nanotube that is functionalized with a catalyticparticle simultaneously including mesopores and macropores which areformed during the simultaneous high-temperature decomposition of a metalparticle and a spherical polymer which are encapsulated in a protein andembedded in an electrospun metal salt precursor/polymer compositenanotube as sacrificial templates and has a double average surface poredistribution, a gas sensor member and a gas sensor which use the same,and manufacturing methods thereof.

BACKGROUND

Recently, as the social interest in the health care has increased, thedevelopment of gas sensors that are based on a metal oxide semiconductorand intended to detect harmful environmental gases have been activelycarried out for the detection of volatile organic compound gases in theexhaled breath and the measurement of air quality. Such gas a sensorbased on a metal oxide semiconductor senses a gas by measuring theelectrical resistance which changes in the adsorption and desorptionprocesses of a specific gas to be detected by the interaction with theoxygen ion adsorbed onto the metal oxide surface. In particular, a metaloxide gas sensor has an advantage of being easily miniaturized, and thusa study to mount the gas sensor to a portable device or a wearabledevice has recently been attempted from a commercial point of view. Inaddition, it also has an advantage that the price is inexpensive, andthus it is widely applied in society as a harmful environment gasdetector, a breathalyzer, an air pollution detector, an anti-terrorismgas sensor, and the like. Particularly, in recent years, a possibilityhas been proposed that a variety of diseases such diabetes, nephritis,asthma, halitosis, lung cancer can be diagnosed by detecting asignificantly small amount of a volatile organic compound gas, such asacetone, ammonia, nitrogen monoxide, hydrogen sulfide, or toluene, thatis contained in the exhaled breath and associated with the biologicalmetabolism using the superior detection capability of the metal oxidesensor. In practice, however, it is required to be able to sense such abiomarker gas having a significantly low concentration in a range offrom 10 ppb (parts per billion) to 10 ppm (parts per million) with ahigh speed of a few seconds and a high sensitivity in order to diagnosethe disease at an early stage by using the biomarker gas. In particular,it is required to react with a specific target biomarker gas among thethousands of mixed gases contained in the exhaled breath with a highsensitivity, and thus it is significantly important to develop a sensingmaterial exhibiting high selectivity to the gas to be measured.

In order to equip a gas sensor based on a metal oxide semiconductor withultrahigh sensitivity/high selectivity, the development of the gassensor based on a variety of nanostructures including nanoparticles,nanofibers, and nanotubes have recently been studied. As mentionedabove, the metal oxide-based gas sensor utilizes the surface reaction ofthe sensing material with the gas to be detected, and thus a highersensitivity is expected as the surface area of the sensing material onwhich the reaction with the gas molecule to be detected takes place iswider. From this point of view, the nanostructure sensing materialexhibits excellent gas detection characteristics since it has a relativewider area for the reaction with a gas as compared with a thick filmmaterial or a thin film material, and the nanostructure sensing materialhas a porous structure through which the gas molecules can sufficientlyrapidly diffuse into the sensing material and thus high-speed responsecharacteristics can be induced. In particular, in the case of aone-dimensional porous metal oxide nanotube having mesopores andmacropores, the surface area can be expected to be from 2 to 10 timesnanofibers having a thin film structure, thus high detectioncharacteristics are expected, and pores having various sizes aredistributed on the tube surface, thus the gas molecules freely moves ascompared with the dense nanofiber and nanotube structures and thecharacteristics of the sensor can be maximized. Additionally, thecatalytic effect can be maximized even with a small amount of catalystif the catalytic nanoparticles are uniformly loaded on theone-dimensional porous nanotube without aggregation with one another. Inaddition, in order to maximize the catalytic effect, rather than astructure in which the catalyst is embedded in a dense sensing materialso as not to be able to react with the gas, it is ideal that the sensingmaterial is functionalized such that the catalyst is exposed to thesurface thereof and thus the catalytic reaction with the gas ismaximized. Such catalysts are largely classified into two types, andthere are a metal catalyst such as platinum (Pt) or gold (Au) used in achemical sensitization method in which the characteristics of the gassensor are enhanced by increasing the concentration of the gasparticipating in the surface reaction by the use such a metal catalystand a metal catalyst such as palladium (Pd), nickel (Ni), cobalt (Co),or silver (Ag) used in an electronic sensitization method in which thesensitivity is improved by a change in oxidation state due to theformation of a metal oxide such as PdO, NiO, Co₂O₃, or Ag₂O.

As described above, although studies to utilize sensing materials formedby loading various nanoparticle catalysts together with the developmentof various nanostructures have been continued, it is the reality that asensing material that is based on a oxide material semiconductor and canprecisely sense a trace amount, less than hundreds ppb, of gas at a highspeed has not yet been commercialized, and it is significantly importantto develop a sensing material which can sense a trace amount of gas andto clearly recognize the pattern of the detected result by impartingselectivity to various kinds of gases for the realization of a exhaledbreath sensor to diagnose the disease at the early stage.

From the viewpoint of the synthesis of a sensing material having ananostructure, a number of studies on the method to manufacturenanostructures through a chemical vapor deposition method, a physicaldeposition method, and a chemical growth method have been carried out.However, these methods include a complicated and cumbersomemanufacturing process upon the synthesis of nanostructures, and thusthere are problems such as difficulties in mass production, an expensiveprocess cost, and a long processing time, which are a major challenge tocommercialization.

In addition, from the viewpoint of the nanoparticle catalyst to beloaded to the sensing material, the most effective catalytic action isinduced when the catalyst is uniformly dispersed without aggregation inthe entire area of the sensing material. In this respect, it isdifficult to optimize the sensing characteristics since the aggregationof nanoparticles is hardly avoided during the synthesis of nanoparticlesutilizing the polyol process and the loading by the mixing of thecatalyst particles and the sensing material which are widely used in theconventional sensor field.

In order to overcome these disadvantages in the conventional sensorsynthesis, an ideal nanostructure that is formed by a simple andeffective method, has a wide surface area, and includes both mesoporesand macropores which lead rapid diffusion and reaction of the gas and aprocess technology which can functionalize the sensor with ananoparticle catalyst having a nano-size by thoroughly dispersingwithout aggregation are required. In addition, a process technologywhich satisfies the two matters described above at the same time andthus contribute to the development of a sensor which can selectivelysense a significantly amount of biomarker gas contained in the actualhuman exhaled breath, recognize the pattern, and ultimately distinguishthe patient with the disease.

SUMMARY

Embodiments of the inventive concepts provide method for synthesizing aone-dimensional porous metal oxide nanotube through electrospinning, inwhich a spherical polymer colloid which plays a role of the sacrificialtemplate and forms macropores is dispersed in an electrospinningsolution, macropores (50 nm to 300 nm) are formed on the nanotubesurface via thermal decomposition of the spherical polymer template(>200 nm) by a high-temperature heat treatment after electrospinning,and sequentially, mesopores (0.1 nm to 50 nm) are formed on the nanotubesurface through the macropores covering effect and thermal decompositionof the protein template (12 nm) by diffusion of the metal oxidegenerated when the tube is formed at the same time. Embodiments of theinventive concepts also provide a method for manufacturing aone-dimensional porous metal oxide nanotube which is uniformly loadedwith a nanoparticle catalyst and has a double average pore distributioncomposed of macropores (50 nm to 300 nm) and mesopores (0.1 nm to 50 nm)through electrospinning, in which a highly dispersible protein-basednanoparticle catalyst is dispersed in an electrospinning solution.

In particular, a technique for synthesizing a sensing materialexhibiting high sensitivity and high selectivity and a catalystuniformly distributed on a one-dimensional porous metal oxide nanotube,in which a polymer sacrificial template having a size of 200 nm or moreis used, macropores (50 nm-300 nm) are formed on the surface of a fiberthrough decomposition of the polymer template by the high temperatureheat treatment, sequentially, in order to form a metal oxide nanotube,the metal oxide diffuses toward the macropores formed on the surface sothat a part of the macropores is filled and mesopores having a sizedistribution of from 0.1 nm to 50 nm are additionally formed on thenanotube surface, and uniform distribution of the catalyst and theformation mesopores are facilitated as a protein-based highlydispersible nanoparticle catalyst is used, and an application techniqueof a gas sensor using the same are proposed. The protein template theso-called apoferritin that is used in the inventive concept is aspherical hollow protein material having an empty space of about 8 nm,and thus it is possible to provide a method for synthesizing a metaloxide nanotube containing a nanoparticle catalyst throughelectrospinning, in which a nanoparticle catalyst is embedded in theempty space of the apoferritin protein and the nanotube isfunctionalized with the apoferritin particles embedding the nanoparticlecatalyst.

In particular, a technique for synthesizing a ultra-sensitive nanotubesensing material which can satisfy an increase in specific surface areato be an important indicator of the gas sensing characteristics and acatalytic effect at the same time as a metal oxide nanotube structurehaving a large surface area is synthesized through the Ostwald ripeningphenomenon although a metallic nanoparticle catalyst is contained afterthe high-temperature heat treatment and the nanoparticle catalyst isalso uniformly dispersed on the shell constituting the nanotube, anapplication technique of a gas sensor using the same are proposed.

In order to solve the problem of the prior art, it is intended toprovide a gas sensor member capable of detecting a trace amount of gasby easily synthesizing a metal oxide nanotube structure in whichnanoparticle catalysts having a significantly small (1 nm to 3 nm) sizeare uniformly dispersed and loaded on the outside and inside of a metaloxide without being aggregated with one another, and at the same time, agreat number of mesopores (0.1 nm to 50 nm) and macropores (50 nm to 300nm) are formed by single electrospinning and the post-heat treatment, agas sensor using the same, and manufacturing methods thereof.

One aspect of embodiments of the inventive concept is directed toprovide a method for manufacturing a sensing material including aone-dimensional porous metal oxide nanotube on which a nanoparticlecatalyst is uniformly loaded and mesopores and macropores are formed atthe same time and a gas sensor member using the same, using a singleprocess in which a nanoparticle catalyst exhibiting superiordispersibility due to the surface charge properties is synthesized and aspherical polymer sacrificial template colloid exhibiting excellentdispersibility is applied to an electrospinning solution at the sametime. The method for manufacturing a sensing material and a gas sensormember using the same according embodiments of the inventive conceptincludes a method for manufacturing a catalyst-metal oxide nanotubecomposite sensing material having a double surface pore distribution fora gas sensor capable of detecting a harmful environmental gas and abiomarker gas for diagnosis of a disease, including: (a) a step ofsynthesizing a dispersion in which a metallic nanoparticle catalystencapsulated in a protein and embedded in an inner hollow structure ofan apoferritin is uniformly dispersed; (b) a step of preparing anelectrospinning solution by mixing the dispersion in which a metallicnanoparticle catalyst encapsulated in a protein and embedded in an innerhollow structure of an apoferritin is uniformly dispersed with adispersion of a spherical polymer sacrificial template and mixing themixed dispersion with a solution in which a metal oxide precursor (metalsalt precursor) and a polymer are dissolved; (c) a step of forming acomposite nanofiber in which at least one or more spherical polymersacrificial templates and a plurality of the metallic nanoparticlecatalysts embedded in an inner hollow structure of the apoferritinprotein are uniformly distributed on the surface and in the inside ofthe metal oxide precursor/polymer composite nanofiber from theelectrospinning solution using an electrospinning method; (d) a step offorming macropores (50 nm to 300 nm) on the fiber surface throughthermal decomposition of the polymer sacrificial template by ahigh-temperature heat treatment, sequentially forming mesopores having asize distribution of from 0.1 nm to 50 nm by filling a part of themacropores with the metal oxide which diffuses toward the macroporesformed on the surface so as to form a metal oxide nanotube, anduniformly loading the nanoparticle catalyst based on the protein in thecomposite nanofiber to the porous nanotube by allowing the nanoparticlecatalyst to diffuse outward; and (e) a step of fabricating a resistancechange-type semiconductor gas sensor by dispersing or grinding theporous metal oxide nanotube in which the nanoparticle catalyst having adouble surface pore distribution is uniformly loaded in the inside andon the inner surface and outer surface of the shell constituting thenanotube and mesopores and macropores are formed and coating it on asensor electrode for a semiconductor type gas sensor using at least onecoating method among drop coating, spin coating, ink-jet printing, anddispensing.

Here, in the step (a), the apoferritin is a protein that is obtained byremoving the iron component from a ferritin protein which is present inthe mucosal cells of the small intestine and contains an iron componentand has a (hollow) structure with an empty space of about 8 nm, and theoverall size of the apoferritin is 12 nm. A variety of metal ions candiffuse and enter the inside of the hollow structure of the apoferritin,and various kinds of nanoparticle catalyst can be easily synthesizedthrough reduction. The kind and form of the metal salt which can besubstituted into the inside of the apoferritin may significantly vary,and representative examples of the catalyst in a salt form may includecopper(II) nitrate, copper(II) chloride, cobalt(II) nitrate, cobalt(II)acetate, lanthanum(III) nitrate, lanthanum(III) acetate, platinum(IV)chloride, platinum(II) acetate, gold(I, III) chloride, gold(III)acetate, silver chloride, silver acetate, Iron(III) chloride, Iron(III)acetate, nickel(II) chloride, nickel(II) acetate, ruthenium(III)chloride, ruthenium acetate, iridium(III) chloride, iridium acetate,tantalum(V) chloride, and palladium(II) chloride. The kind of the metalsalt is not particularly limited as long as it is in the form of a saltcontaining a metal ion, single metal particles are formed in the hollowportion of the apoferritin in the case of using a single metal, and thebinding force between the same kind of metals is strong so that thephases have a segregated form and bonding between heterogeneous metalsis facilitated and thus it is possible to synthesize a nanoparticlecatalyst formed in the hollow portion of the apoferritin in a metalalloy form having a strong binding force in the case of synthesizingusing two metal salts at the same time. In the case of the nanoparticlecatalyst that is synthesized as being embedded in the hollow structureof the apoferritin, the surface thereof is surrounded by a proteinhaving a surface charge, and thus the nanoparticle catalysts canmaintain an effectively dispersed state without aggregation with oneanother.

In addition, the step (b) is a step of preparing an electrospinningsolution used in electrospinning, and the electrospinning solution canbe prepared by dissolving a polymer which acts as the template foreffectively synthesizing the nanofiber during the electrospinning and ametal oxide precursor in a solvent. Examples of the representativepolymer used at this time may include polymethyl methacrylate (PMMA),polyvinylpyrrolidone (PVP), polyvinyl acetate (PVAc), polyvinyl alcohol(PVA), polyacrylonitrile (PAN), polyethylene oxide (PEO), polypropyleneoxide (PPO), polyethylene oxide copolymer, polypropylene oxidecopolymer, polycarbonate (PC), polyvinylchloride (PVC),polycaprolactone, and polyvinylidene fluoride, and examples of therepresentative metal salt may include acetate, chloride,acetylacetonate, nitrate, methoxide, ethoxide, butoxide, isopropoxide,and sulfide which contain a metal salt. Additionally, it is possible toprepare an electrospinning solution in a colloidal form by uniformlydispersing a solution of the nanoparticle catalyst that is synthesizedin the step (a) and encapsulated in the apoferritin protein andspherical polymer sacrificial template colloids which exhibits excellentdispersibility in an electrospinning solution. The spherical polymersacrificial template used for the formation of macropores refers to atemplate that can be removed during the high-temperature heat treatment,and the kind of the template is not particularly limited. Specifically,it may be one kind or a mixture of two or more kinds selected frompolymethyl methacrylate (PMMA), polyvinylpyrrolidone (PVP), polyvinylacetate (PVAc), polyvinyl alcohol (PVA), polystyrene (PS),polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyacrylicacid (PAA), polydiallydimethylammonium chloride (PDADMAC), orpolystyrene sulfonate (PSS). In addition, the sacrificial template has asize in the range of from 50 nm to 1 μm, the sacrificial template ispreferably dispersed without being decomposed when mixed with anelectrospinning solution, and a polymer colloid that is insoluble in asolvent since a charged ion or a charged anionic or cationic surfactantis formed on the surface of the colloid may be used as the sacrificialtemplate although the sacrificial colloid is a polymer soluble in thesolvent.

In addition, the step (c) is a step of synthesizing a metal salt/polymercomposite nanofiber on which the metallic nanoparticles (metallicnanoparticle catalyst) and the spherical polymer sacrificial template(polymeric beads) are uniformly loaded using the electrospinning method.The composite nanofiber has a rugged shape due to the polymersacrificial template embedded therein.

In the step (d), the polymer constituting the polymer/metal oxideprecursor composite nanofiber is decomposed and removed through thehigh-temperature heat treatment, and at the same time, the apoferritinprotein shell surrounding the nanoparticle catalyst and the sphericalpolymer sacrificial template are removed. Specifically, the macroporesformed on the nanofiber surface are generated as the polymer having asize of 200 nm or more is decomposed through the high-temperature heattreatment, sequentially, the metal oxide is crystallized and diffusesoutward to partially cover the macropores in the course of the formationof metal oxide nanotubes, as a result, a number of mesopores are formed.

In addition, the formation of mesopores is also attributed to thedecomposition of the apoferritin particles that are densely gathered inbetween the plurality of polymer sacrificial templates. In particular,the heating rate plays an important role in the formation of thenanotube structure. In the case of conducting the heat treatment at ahigh heating rate of 10° C./min, it is possible to more effectivelysynthesize the one-dimensional porous metal oxide nanotube having adouble pore distribution (mesopores and macropores coexist) in which themetallic nanoparticle catalyst obtained by the decomposition of theapoferritin protein having the nanoparticle catalyst formed inside thehollow structure is included in the metal oxide nanotube structure. Onthe other hand, the nanotube structure may not be formed in the case ofconducting the heat treatment at a relatively low heating rate of 4°C./min.

The step (e) may be a step of coating a dispersion prepared bydispersing the one-dimensional porous metal oxide nanotube having adouble pore distribution obtained in the step (d) in a solvent on asensor electrode (an alumina insulating substrate on which parallelelectrodes capable of measuring the electrical conductivity and theelectrical resistance are formed) that is prepared in advance using acoating method such as drop coating, spin coating, ink-jet printing, ordispensing. Here, the coating method is not particularly limited as longas it is a method by which the one-dimensional porous metal oxidenanotube which contains a nanoparticle catalyst and has a double poredistribution can be uniformly coated.

In the one-dimensional porous metal oxide nanotube structure that isthus fabricated and has a double pore distribution, the thicknessbetween the inner and outer walls can be determined in a length range offrom 10 nm to 50 nm, and the diameter of the nanotube may be in a lengthrange of from 50 nm to 5 μm. The length of the nanotube may be in alength range of from 1 μm to 100 μm. In addition, the nanotube structureincludes a plurality of mesopores in a range of from 0.1 nm to 50 nm andmacropores in a size range of from 50 nm to 300 nm on the outer surfaceof the tube.

Another aspect of embodiments of the inventive concept is directed toprovide a method for manufacturing a sensing material including ananoparticle catalyst which has a large surface area and is uniformlydistributed at the same time as a nanoparticle catalyst exhibitingsuperior dispersibility is synthesized and is uniformly loaded in theinside and on the outside of the one-dimensional metal oxide nanotubesynthesized by an easy single process and a gas sensor member using thesame. This method relates to a method for manufacturing a gas sensormember in which catalyst particles exhibiting high dispersibility issingly loaded on a nanotube without mixing the catalyst particles withthe polymer template described above. The method for manufacturing asensing material and a gas sensor member using the same according toembodiments of the inventive concept includes (a) a step of synthesizinga nanoparticle catalyst using an apoferritin; (b) a step of preparing ametal oxide precursor/polymer mixed electrospinning solution containingthe nanoparticle catalyst contained(embedded) in the hollow structure ofthe apoferritin; (c) a step of forming a metal oxide precursor/polymercomposite nanofiber containing a nanoparticle catalyst embedded in thehollow structure of the apoferritin on the surface or in the inside ofthe metal oxide precursor/polymer composite nanofiber using anelectrospinning method; (d) a step of removing the apoferritin of aprotein component encapsulating the nanoparticle catalyst and thepolymer substance through thermal decomposition by a high-temperatureheat treatment at a high heating rate and forming a one-dimensionalmetal oxide nanotube containing the nanoparticle catalyst in the shellthrough the Ostwald ripening; and (e) a step of dispersing the metaloxide nanotube substance loaded with the metallic nanoparticle catalystand coating it on an electrode for a gas sensor by drop to fabricating agas sensor, and the method may further include (f) a step of fabricatingat least two or more kinds of metal oxide nanotube sensors loaded withthe nanoparticle catalyst in combination of different nanoparticlecatalysts or different metal oxide sensing materials to constitute asensor array. The method includes a method for manufacturing a structurecontaining a nanoparticle catalyst that is uniformly dispersed on thesurface and in the inside of the one-dimensional nanotube through singleelectrospinning according to the process described above.

Here, the step (a) is the same as the step of synthesizing thenanoparticle catalyst in the process to manufacture a nanotube havingmesopores and macropores.

In addition, the step (b) is a step of preparing an electrospinningsolution used in electrospinning, and the electrospinning solution canbe prepared by dissolving a polymer which acts as the template foreasily forming the nanofiber and a metal oxide which acts as theprecursor in a solvent. Specific examples of the polymer may includepolymethyl methacrylate (PMMA), polyvinylpyrrolidone (PVP), polyvinylacetate (PVAc), polyvinyl alcohol (PVA), polyacrylonitrile (PAN),polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene oxidecopolymer, polypropylene oxide copolymer, polycarbonate (PC),polyvinylchloride (PVC), polycaprolactone, and polyvinylidene fluoride,and examples of the representative metal salt may include acetate,chloride, acetylacetonate, nitrate, methoxide, ethoxide, butoxide,isopropoxide, and sulfide which contain a metal salt. Additionally, itis possible to prepare an electrospinning solution by adding theapoferritin protein having the nanoparticle catalyst that is synthesizedin the step (a) formed in the hollow structure thereof. In the case ofpreparing the electrospinning solution, the concentration of theapoferritin protein having the nanoparticle catalyst formed in thehollow structure thereof can be variously controlled in a range of from0.001 wt % to 50 wt %. The content of the nanoparticle catalystcontained in the shell of the metal oxide nanotubes is controlleddepending on to the concentration of the apoferritin protein.

In addition, the step (c) is a step of synthesizing a metal salt/polymercomposite nanofiber using an electrospinning method, and the apoferritinprotein having the nanoparticle catalyst that is synthesized in the step(a) formed in the hollow structure thereof is uniformly distributed inthe inside of the metal oxide precursor/polymer composite nanofiber dueto excellent dispersibility of the apoferritin protein.

In addition, in the step (d), the polymer constituting the polymer/metaloxide precursor composite nanofiber the is decomposed and removedthrough a high-temperature heat treatment, and the metal oxide precursorundergoes the oxidation and the Ostwald ripening, thus it is possible toform a metal oxide nanotube structure having a one-dimensional structureand contains a nanoparticle catalyst. In particular, the heating rateplays an important role in the formation of the nanotube structure. Inthe case of conducting the heat treatment at a high heating rate of 10°C./min, it is possible to more effectively synthesize the metal oxidenanotube containing the metallic nanoparticle catalyst obtained by thedecomposition of the apoferritin protein having the nanoparticlecatalyst formed inside the hollow structure in the shell structure. Onthe other hand, the nanotube structure may not be formed in the case ofconducting the heat treatment at a relatively low heating rate of 4°C./min.

The step (e) may be a step of coating a dispersion prepared bydispersing the polycrystalline metal oxide nanotubes loaded withnanoparticle catalyst obtained in the step (d) in a solvent on a sensorelectrode (an alumina insulating substrate on which parallel electrodescapable of measuring the electrical conductivity and the electricalresistance are formed) that is prepared in advance using a coatingmethod such as drop coating, spin coating, ink-jet printing, ordispensing. Here, the coating method is not particularly limited as longas it is a method by which the polycrystalline nanotube containing themetallic nanoparticle catalyst obtained by the decomposition of theapoferritin protein having the nanoparticle catalyst formed inside thehollow structure in the shell structure can be uniformly coated.

In addition, here, in the step (f), the sensor that is fabricated in thestep (e) and has a metal oxide nanotube structure containing ananoparticle catalyst may be formed into a sensor array constituted bytwo or more kinds of composite sensing materials including various kindsof nanoparticle catalyst-metal oxide nanotube composite sensingmaterials having a combination of different nanoparticle catalysts anddifferent metal oxide nanotubes having a one-dimensional structure.

In the one-dimensional metal oxide nanotube structure fabricated above,the thickness between the inner and outer walls can be determined in alength range of from 10 nm to 50 nm, and the length of the nanotube maybe in a length range of from 1 μm to 500 μm.

Here, in the case of the sensing material fabricated above, thenanoparticle catalyst is intensively and uniformly contained in theshell portion constituting the metal oxide nanotube so that thecatalytic properties and the sensitivity of the sensing material can bemaximized at the same time.

The weight ratio of the nanoparticle catalyst in the nanoparticlecatalyst-metal oxide nanotube composite sensing material fabricated bythe method described above can be selected from a range of from 0.001 wt% to 50 wt % with respect to the weight of the metal oxide nanotube, andthe nanoparticle catalyst-metal oxide nanotube composite sensingmaterial can sense specific gases contained in the exhaled breath of ahuman body to determine the presence or absence of disease and also cansense the harmful gas in the indoor and outdoor environment.

Embodiments of the inventive concept is characterized in thatnanoparticle catalysts having a size of from 1 nm to 3 nm are formedusing a protein template exhibiting excellent dispersibility due to therepulsive force therebetween since the surface of the protein templateis positively charged, the nanoparticle catalysts thus formed are mixedinto the electrospinning solution, a spherical template colloid is alsomixed into the electrospinning solution, and the spherical template andthe catalyst are uniformly distributed on the composite nanofiber byelectrospinning. In addition, embodiments of the inventive concept ischaracterized in that the Ostwald ripening and the polymer decompositiondue to a high heating rate in the high-temperature heat treatment areused to form a one-dimensional porous metal oxide structure on which thenanoparticle catalyst is uniformly loaded and in which two kinds ofpores and distributed on the metal oxide surface. An effect ofdisclosing a gas sensor member and a gas sensor which can bemanufactured on a large scale and manufacturing methods thereof isobtained by controlling the shape which increase the catalytic effectand reaction surface area to be important factors in the gas sensingcharacteristics so as to exhibit high sensitivity characteristics to beable to detect a trace amount of gas of about 10 parts per billion,variously changing the material composition so as to exhibit excellentselectivity to be able to detect a variety of gases, controlling theelectrospinning and the heat treatment, and controlling the shape of thenanotube that is loaded with a catalyst and includes a number of poresthrough a simple process at the same time.

According to embodiments of the inventive concept, a spherical polymersacrificial template is used when synthesizing a one-dimensional porousmetal oxide nanotube having a plurality of circular or ellipticalmesopores and a number of macropores, a one-dimensional porous nanotubestructure having mesopores and macropores on the surface of thenanotubes is formed by a single process using the time differencebetween the polymer decomposition and the crystallization and diffusionof the metal oxide, a porous rube structure having a specific surfacearea to be several ten times as large as that of a general thin-filmstructure and to be several times as large as that of a dense tubestructure using the protein templates that are densely gathered inbetween a plurality of polymers. An effect of improving the sensingcharacteristics is obtained as gas molecules smoothly flow through thepores present on the tube surface and the adsorption and desorption ofthe gas molecules to the metal oxide nanotube surface is facilitated. Inaddition, the nanoparticle catalyst encapsulated in the apoferritin iscontained in the electrospinning solution, the protein encapsulating thenanoparticle catalyst is all removed through the high-temperature heattreatment after being electrospun, the nanoparticles having a size in arange of from 1 nm to 3 nm are exposed on a newly formed surface bybeing diffused through the inner wall and the outer wall and the poresof the porous nanotube during the Ostwald ripening, and thus thecatalytic reaction effect can be maximized. The protein having an innerhollow size of 8 nm can additionally form ultra-micro pores on thesurface of the nanotubes while being removed. As described above, aneffect of disclosing a gas sensor member and a gas sensor which can bemanufactured on a large scale, exhibit high sensitivity characteristicsto be able to detect a trace amount of gas, and exhibit excellentselectivity to be able to detect a specific gas, and manufacturingmethods thereof is obtained as the sensing characteristics are maximizedthrough the shape control and catalytic reaction effect of the gassensor member.

According to embodiments of the inventive concept, as the nanotubestructure is formed by a single process by controlling the heattreatment conditions upon fabricating a hollow fiber having aone-dimensional metal oxide nanotube structure, the hollow fiber has aspecific surface area to be 6 times as large as that of a generalthin-film structure, the movement of the gas into the tube isfacilitated, and an effect of improving the sensitivity for a smallamount of gas is obtained. In addition, it is possible to maximize thecatalytic reaction by manufacture a gas sensor using a sensing materialin which the nanoparticle catalysts are uniformly loaded on the innerwall and the outer wall of the metal oxide nanotube without aggregationas the nanoparticle catalysts encapsulated inside the apoferritin arecontained in the electrospinning solution and subjected to ahigh-temperature heat treatment after being electrospun. As describedabove, an effect of disclosing a gas sensor member and a gas sensorwhich can be manufactured on a large scale, exhibit high sensitivitycharacteristics to be able to detect a trace amount of gas, and exhibitexcellent selectivity to be able to detect a specific gas, andmanufacturing methods thereof is obtained by maximizing the surface areaand catalytic reaction effect of the gas sensor member.

BRIEF DESCRIPTION OF THE FIGURES

The inventive concepts will become more apparent in view of the attacheddrawings and accompanying detailed description.

FIG. 1 is a schematic diagram of a gas sensor member using aone-dimensional porous metal oxide nanotube which is uniformly loadedwith a nanoparticle catalyst and includes a plurality of circular orelliptical mesopores and macropores according to an embodiment of theinventive concept.

FIG. 2 is a flow chart of a method of manufacturing a gas sensor using aone-dimensional porous metal oxide nanotube structure which contains ananoparticle catalyst synthesized using an apoferritin and includes aplurality of circular or elliptical mesopores and macropores accordingto an embodiment of the inventive concept.

FIG. 3 is a diagram illustrating a process of manufacturing aone-dimensional porous metal oxide nanotube structure which contains ananoparticle catalyst, includes a plurality of circular or ellipticalpores, and has a double pore distribution using an electrospinningmethod according to an embodiment of the inventive concept.

FIG. 4 is a diagram illustrating a principle that mesopores are formedon the nanotube surface by the spherical sacrificial template and thecrystallization and diffusion of the metal oxide according to anembodiment of the inventive concept.

FIG. 5 is a diagram illustrating a principle that mesopores are formedby a protein having hollow structure according to an embodiment of theinventive concept.

FIG. 6 is a scanning electron microscope (SEM) image of a sphericalpolymer sacrificial template which plays a role of the sacrificialtemplate according to an embodiment of the inventive concept.

FIGS. 7A and 7B are transmission electron microscope (TEM) images ofapoferritin particles encapsulating a Pt nanoparticle catalyst accordingto Embodiment Example 1 of the inventive concept, FIG. 7C illustrateszeta potential data to analyze the surface charge of the particles, andFIG. 7D illustrates the size distribution of Pt nanoparticle catalysts,respectively.

FIG. 8 is a SEM image of a nanofiber obtained by electrospinning a metaloxide precursor/polyvinylpyrrolidone (PVP) composite electrospinningsolution containing an apoferritin protein which contains a Ptnanoparticle catalyst and has a hollow structure and a spherical polymersacrificial template according to an embodiment of the inventiveconcept.

FIGS. 9A and 9B are SEM images of a one-dimensional porous metal oxidenanotube which contains a Pt nanoparticle catalyst obtained byelectrospinning an electrospinning solution prepared by adding Ptnanoparticles synthesized using tin oxide precursor/polyvinylpyrrolidone(PVP) and an apoferritin and a spherical polymer sacrificial templatecolloid and conducting a high-temperature heat treatment and includesmesopores and macropores according to Embodiment Example 2 of theinventive concept.

FIGS. 10A, 10B, and 10C are TEM images of a one-dimensional porous metaloxide nanotube which contains a Pt nanoparticle catalyst and includes aplurality of mesopores and macropores according to Embodiment Example 2of the inventive concept, FIG. 10D illustrates the selected areaelectron diffraction (SAED) pattern, and FIG. 10E is energy dispersiveX-ray spectrometer (EDS) images.

FIGS. 11A and 11B are the thermogravimetric analysis graph and thephotoelectron spectroscopic (XPS) analysis graph of a one-dimensionalporous metal oxide nanotube which contains a Pt nanoparticle catalystand includes a plurality of mesopores and macropores according toEmbodiment Example 2 of the inventive concept, respectively.

FIG. 12 is a SEM image of a metal oxide nanotube obtained byelectrospinning a metal oxide precursor/polyvinylpyrrolidone (PVP)composite electrospinning solution and by conducting a high-temperatureheat treatment under a high heating rate condition according toComparative Example 1 of the inventive concept.

FIGS. 13A and 13B are SEM images of a one-dimensional porous metal oxidenanotube which is obtained by subjecting a metal oxideprecursor/polyvinylpyrrolidone (PVP) composite nanofiber containing aspherical polymer sacrificial template to a high-temperature heattreatment under a high heating rate condition and has a double poredistribution according to Comparative Example 2 of the inventiveconcept.

FIG. 14A is a graph illustrating the acetone (100 ppb to 5 ppm) responseproperties of a one-dimensional porous metal oxide nanotube whichcontains a Pt nanoparticle catalyst and includes a plurality ofmesopores and macropores according to Embodiment Example 2 of theinventive concept, a pure tin oxide nanotube structure according toComparative Example 1, and a one-dimensional porous tin oxide nanotubestructure having a double pore distribution composed of a plurality ofcircular and elliptical pores according to Comparative Example 2 at 350°C. FIG. 14B is a graph illustrating the acetone detection limitcharacteristics of a one-dimensional porous metal oxide nanotube sensingmaterial which contains a Pt nanoparticle catalyst and includes aplurality of mesopores and macropores according to Embodiment Example 2of the inventive concept.

FIG. 15 is a graph illustrating the response properties of a gas sensorhaving a one-dimensional porous metal oxide nanotube structure whichcontains a Pt nanoparticle catalyst and includes a plurality ofmesopores and macropores according to Embodiment Example 2 of theinventive concept to biomarker gases such as acetone (CH₃COCH₃), toluene(C₆H₅CH₃), hydrogen sulfide (H₂S), nitrogen monoxide (NO), carbonmonoxide (CO), pentane (C₅H₁₂), and ammonia (NH₃) at 1 ppm and 350° C.

FIG. 16 is a diagram illustrating a process to collect the exhaledbreath of 10 healthy people and a process to prepare the exhaled breathof simulated diabetic patients so that the exhaled breath is adjustedsimilar to the exhaled breath of a real diabetic patient according to anembodiment of the inventive concept.

FIG. 17 is a graph illustrating the results of principal componentanalysis (PCA) on the exhaled breath of healthy people and the exhaledbreath of simulated diabetic patients using arrays of the sensingmaterials produced according to embodiments of the inventive concept,which shows that the exhaled breath of simulated diabetic patients isdistinguished from that of healthy people.

FIG. 18 is a schematic diagram of a gas sensor member in which ananoparticle catalyst is uniformly loaded on the inside and outside of aone-dimensional metal oxide nanotube according to Embodiment Example 4of the inventive concept.

FIG. 19 is a flow chart of a manufacturing method of a gas sensor usinga metal oxide nanotube structure containing a nanoparticle catalystsynthesized using an apoferritin according to Embodiment Example 4 ofthe inventive concept.

FIG. 20 is a diagram illustrating a manufacturing process of aone-dimensional metal oxide nanotube structure containing a nanoparticlecatalyst using an electrospinning method according to Embodiment Example4 of the inventive concept.

FIG. 21 is SEM images of a nanofiber obtained by electrospinning a tinoxide precursor/polyvinylpyrrolidone (PVP) composite electrospinningsolution containing an apoferritin protein having a Pt nanoparticlecatalyst and an Au nanoparticle catalyst encapsulated in the inside ofthe hollow structure according to an embodiment of the inventiveconcept.

FIG. 22 is a SEM image of a tin oxide nanofiber obtained byelectrospinning a tin oxide precursor/polyvinylpyrrolidone (PVP)composite electrospinning solution and conducting a high-temperatureheat treatment according to Comparative Example 3 of the inventiveconcept.

FIG. 23 is a SEM image of a tin oxide nanotube obtained byelectrospinning a tin oxide precursor/polyvinylpyrrolidone (PVP)composite electrospinning solution and conducting a high-temperatureheat treatment under a high heating rate condition according toComparative Example 4 of the inventive concept.

FIG. 24 is TEM images of apoferritin particles encapsulating a Ptnanoparticle catalyst and apoferritin particles encapsulating an Aunanoparticle catalyst according to Embodiment Example 3 of the inventiveconcept.

FIG. 25 is SEM images of a tin oxide nanotube containing a Ptnanoparticle catalyst and a tin oxide nanotube containing an Aunanoparticle catalyst obtained by electrospinning an electrospinningsolution prepared by adding tin oxide precursor/polyvinylpyrrolidone(PVP) and Pt nanoparticles and Au nanoparticles synthesized using anapoferritin and conducting a high-temperature heat treatment under ahigh heating rate condition according to Embodiment Example 4 of theinventive concept.

FIG. 26 is TEM and EDS images of a tin oxide nanotube structurecontaining a Pt nanoparticle catalyst according to Embodiment Example 4of the inventive concept.

FIG. 27 is TEM and EDS images of a tin oxide nanotube structurecontaining an Au nanoparticle catalyst according to Embodiment Example 4of the inventive concept.

FIG. 28 is a graph illustrating the acetone (1 to 5 ppm) responseproperties of a tin oxide nanotube containing a Pt nanoparticle catalystaccording to Embodiment Example 4 of the inventive concept, a pure tinoxide nanotube structure according to Comparative Example 4, and a tinoxide nanotube structure according to Comparative Example 3 at 350° C.

FIG. 29 is a graph illustrating the hydrogen sulfide (1 to 5 ppm)response properties of a tin oxide nanotube containing a Pt nanoparticlecatalyst according to Embodiment Example 4 of the inventive concept, apure tin oxide nanotube structure according to Comparative Example 4,and a tin oxide nanotube structure according to Comparative Example 3 at350° C.

FIG. 30 is a graph illustrating the toluene (1 to 5 ppm) responseproperties of a tin oxide nanotube containing a Pt nanoparticle catalystaccording to Embodiment Example 4 of the inventive concept, a pure tinoxide nanotube structure according to Comparative Example 4, and a tinoxide nanotube structure according to Comparative Example 3 at 350° C.

FIG. 31 is a graph illustrating the response properties of a gas sensorusing tin oxide having a one-dimensional nanotube structure loaded witha Pt nanoparticle catalyst according to Embodiment Example 4 of theinventive concept to biomarker gases such as acetone (CH₃COCH₃), toluene(C₆H₅CH₃), hydrogen sulfide (H₂S), nitrogen monoxide (NO), carbonmonoxide (CO), pentane (C₅H₁₂), and ammonia (NH₃) at 1 ppm and 350° C.

FIG. 32 is a graph illustrating the hydrogen sulfide (1 to 5 ppm)response properties of a tin oxide nanotube containing an Aunanoparticle catalyst according to Embodiment Example 4 of the inventiveconcept, a pure tin oxide nanotube structure according to ComparativeExample 4, and a tin oxide nanotube structure according to ComparativeExample 3 at 350° C.

FIG. 33 is a graph illustrating the response properties of a gas sensorusing tin oxide having a one-dimensional nanotube structure loaded withan Au nanoparticle catalyst according to Embodiment Example 4 of theinventive concept to biomarker gases such as acetone (CH₃COCH₃), toluene(C₆H₅CH₃), hydrogen sulfide (H₂S), ethanol (C₂H₅OH), and ammonia (NH₃)at 1 ppm and 300° C.

DETAILED DESCRIPTION

The inventive concepts will now be described more fully hereinafter withreference to the accompanying drawings, in which exemplary embodimentsof the inventive concepts are shown. The advantages and features of theinventive concepts and methods of achieving them will be apparent fromthe following exemplary embodiments that will be described in moredetail with reference to the accompanying drawings. It should be noted,however, that the inventive concepts are not limited to the followingexemplary embodiments, and may be implemented in various forms.

In addition, in explanation of the present invention, the descriptionsto the elements and functions of related arts may be omitted if theyobscure the subjects of the present invention.

It will be also understood that although the terms first, second, thirdetc. may be used herein to describe various elements, these elementsshould not be limited by these terms. These terms are only used todistinguish one element from another element.

Hereinafter, a gas sensor member using a sensing material in which aone-dimensional porous metal oxide nanotube simultaneously havingmesopores and macropores by the time difference between thedecomposition of the sacrificial polymer and the diffusion of the metaloxide is functionalized with a protein-based highly dispersiblenanoparticle catalyst, a gas sensor, and manufacturing methods thereofwill be described in detail with reference to the accompanying drawings.

Embodiments of the inventive concept relate to a one-dimensional porousnanotube gas sensor member which contains a nanoparticle catalystsynthesized using an apoferritin and in which mesopores (0.1 nm to 50nm) and macropores (50 nm to 300 nm) are formed on the metal oxidenanotube and the nanoparticle catalysts are uniformly distributed on themetal oxide nanotube at the same time by the decomposition of thepolystyrene polymer and the crystallization and diffusion of the metaloxide which sequentially occur during the high-temperature heattreatment of the metal oxide precursor/polymer composite nanofibercontaining spherical polystyrene colloids. In the case of the study onthe existing gas sensor using a metal oxide, studies for improving thesensing characteristics have been carried out in which a structure thatcan react with a large amount of gas is created by increasing thespecific surface area in order to improve the sensing characteristics ofthe metal oxide sensing material, and also studies have been carried outin which the catalytic reaction is promoted by loading a metal or metaloxide catalyst on the sensing material. In other words, it can be seenthat the shape and catalytic activity of the sensing material are twoimportant factors for improving the sensing characteristics. However,the studies that have been carried out so far have a disadvantage thatthe process to increase the specific surface area and the process toload the catalyst on the sensing material are separately required andthe respective processes are fairly complicated. Specifically, theprocess to uniformly synthesize nanoparticle catalysts having a size ofseveral nm requires various pre-treatment processes, in the case ofsynthesizing a metal oxide nanotube or a metal oxide nanotube havingpores, there is a disadvantage that the process is relativelycomplicated and requires a long time and a high cost. In order toovercome these disadvantages and to design an optimum sensing material,in the inventive concept, a nanoparticle catalyst having a uniform sizedistribution of about from 1 nm to 3 nm is easily synthesized using anapoferritin of a protein template, the nanoparticle catalyst is mixedinto a metal oxide precursor/polymer mixed electrospinning solutiontogether with spherical polystyrene colloids having a wide sizedistribution of from 200 nm to 1000 nm, and the nanoparticle catalystand the spherical polystyrene sacrificial template are uniformly loadedon the surface and in the inside of the metal oxide precursor/polymercomposite nanofiber. Moreover, as mesopores (0.1 nm to 50 nm) andmacropores (50 nm to 300 nm) are formed and a one-dimensional porousmetal oxide nanotube structure on which the nanoparticle catalysts areuniformly loaded is formed using the decomposition of the sacrificialpolymer and the crystallization and diffusion of the metal oxide whichsequentially occur during the high-temperature heat treatment of thesynthesized composite nanofiber, it is possible to easily synthesize aone-dimensional porous nanotube sensing material which has a largespecific surface area and a double pore distribution and on which thenanoparticle catalyst is uniformly loaded without aggregation to exhibitmaximized catalytic activity in a large scale by a single process. Here,the mesopores having a size range of from 0.1 nm to 50 nm and themacropores in a range of frim 50 nm to 300 nm which are formed on theinner and outer walls of the nanotube not only increase the surface areaof the nanotube but also maximize the gas flow toward the sensingmaterial. In particular, in order to effectively detect VOCs gases, themesopores having a size range of from 0.1 nm to 50 nm plays an importantrole, the sensing material thus developed has the number of mesopores(0.1 nm to 50 nm) to be several times as many as that of the macropores(50 nm to 300 nm) so as to have an excellent condition as a sensingmaterial. In addition to this, the nanoparticle catalysts that areuniformly distributed on the inner/outer surface of the nanotube and thesurface exposed to the pores without being aggregated with one anothercan maximize the catalytic effect exhibited when a gas reacts with thesensing material in a small amount. The synergistic effect between themorphological concept of the nanotube structure including a number ofpores and the catalytic activity concept of being uniform distributedwithout aggregation can be expected, and thus it is possible tofabricate a highly sensitive sensing material for a gas sensor ascompared to the existing sensing material. In particular, although asacrificial polymer template having a size of several hundred nanometers(nm) is used, it is possible to form mesopores having a size range offrom 0.1 nm to 50 nm and macropores having a size of from 50 nm to 300nm on the nanotube surface. In order to fabricate a gas sensor memberhaving the features as described above, a gas sensor member, a gassensor, and manufacturing methods thereof are implemented by anefficient and easy process.

FIG. 1 is a schematic diagram of a gas sensor member using aone-dimensional porous metal oxide nanotube 100 including a nanoparticlecatalyst 110 and a plurality of mesopores 121 and macropores 131according to Embodiment Example 2 of the inventive concept. Anelectrospinning solution prepared by adding a nanoparticle catalystembedded inside the hollow structure of the apoferritin and a sphericalsacrificial template colloid to a metal oxide precursor/polymer mixedelectrospinning solution, and a metal oxide precursor/polymer compositenanofiber on which the spherical sacrificial template and thenanoparticle catalyst encapsulated in the apoferritin are uniformlyloaded synthesized by electrospinning the electrospinning solution. Thecomposite nanofiber thus formed is subjected to a high-temperature heattreatment, the sacrificial templates and the apoferritin protein shellare removed to form mesopores having a size range of from 0.1 nm to 50nm and macropores having a size of from 50 nm to 300 nm, the metal oxideparticles gather on the fiber surface so as to form mesopores as themacropores are filled by the metal oxide particles, and the nanoparticlecatalysts also gather on the surface so as to be uniformly loaded in theinside and on the outside of the tube structure, whereby aone-dimensional porous nanotube having mesopores and macropores can beformed.

Here, the metal that can be synthesized inside the hollow structure ofthe apoferritin is not particularly limited as long as it is in an ionicform. Specific examples thereof may include copper(II) nitrate,copper(II) chloride, cobalt(II) nitrate, cobalt(II) acetate,lanthanum(III) nitrate, lanthanum(III) acetate, platinum(IV) chloride,platinum(II) acetate, gold(I, III) chloride, gold(III) acetate, silverchloride, silver acetate, iron(III) chloride, iron(III) acetate,nickel(II) chloride, nickel(II) acetate, ruthenium(III) chloride,ruthenium acetate, iridium(III) chloride, iridium acetate, tantalum(V)chloride, and palladium(II) chloride, and it is possible to synthesize ananoparticle catalyst composed of one or two or more of the particlesselected from Pt, Pd, Rh, Ru, Ni, Co, Cr, Ir, Au, Ag, Zn, W, Sn, Sr, In,Pb, Fe, Cu, V, Ta, Sb, Sc, Ti, Mn, Ga, or Ge in an alloy form usingthese precursors. It is possible to use one heterogeneous nanoparticlecatalyst selected form the group consisting of a metal-metal, ametal-metal oxide, or a metal oxide-metal oxide as the alloynanoparticle catalyst. Examples of the representative metal-metal oxidenanoparticle catalyst may include Pt/IrO₂, Pt/RuO₂, Pt/Rh₂O₃, Pt/NiO,Pt/Co₃O₄, Pt/CuO, Pt/Ag₂O, Pt/Fe₂O₃, Au/IrO₂, Au/RuO₂, Au/Rh₂O₃, Au/NiO,Au/Co₃O₄, Au/CuO, and Au/Ag₂O, examples of the metal-metal nanoparticlecatalyst may include Pt-Au, and the metal oxide-metal oxide catalyst maybe a metal oxide-metal oxide catalyst composed of two kinds selectedfrom TiO₂, ZnO, WO₃, SnO₂, IrO₂, In₂O₃, V₂O₃, and MoO₃ of n-type metaloxides and Ag₂O, PdO, RuO₂, Rh₂O₃, NiO, Co₃O₄, CuO, Fe₂O₃, Fe₃O₄, V₂O₅,and Cr₂O₃ of p-type metal oxides. Hence, in the case of synthesizing ananoparticle catalyst using the apoferritin template having a hollowstructure, it is possible not only to synthesize the nanoparticlecatalyst having a constant size distribution but also to control thesize of the nanoparticle catalyst by controlling the amount of the metalprecursor. In addition, the nanoparticle catalysts are encapsulated inthe protein shell, apoferritin, and the apoferritin surface ispositively charged at a pH of from 7 to 8.5 so as to have an advantageof being favorably dispersed in the electrospinning solution withoutbeing aggregated with one another. From the viewpoint of the role ofnanoparticle catalyst acting in the gas sensing material, there are ananoparticle catalyst of a noble metal, such as platinum (Pt) or gold(Au), which exhibits a chemical sensitization effect that theconcentration of adsorbed oxygen ions which involve in the surfacereaction is increased by promoting the decomposition reaction of oxygenmolecule in between the surface of the metal oxide and the air layer anda nanoparticle catalyst which exhibits an electronic sensitizationeffect that a catalytic reaction is caused by the oxidation, such asPdO, Co₃O₄, NiO, Cr₂O₃, CuO, Fe₂O₃, Fe₃O₄, TiO₂, ZnO, SnO₂, V₂O₅, orV₂O₃, which affects the improvement in sensing characteristics.

The spherical polymer sacrificial template used for the synthesis of theone-dimensional porous metal oxide having a double pore distributiondescribed above refers to a template that can be removed during thehigh-temperature heat treatment, and the kind of the template is notparticularly limited. Specifically, it may be one kind or a mixture oftwo or more kinds selected from polymethyl methacrylate (PMMA),polyvinylpyrrolidone (PVP), polyvinyl acetate (PVAc), polyvinyl alcohol(PVA), polystyrene (PS), polyacrylonitrile (PAN), polyvinylidenefluoride (PVDF), polyacrylic acid (PAA), polydiallydimethyμmmoniumchloride (PDADMAC), or polystyrene sulfonate (PSS). In addition, thesacrificial template has a size in the range of from 50 nm to 1 μm, thesacrificial template is preferably dispersed without being decomposedwhen mixed with the electrospinning solution, and a polymer colloid thatis insoluble in a solvent since a charged ion or a charged anionic orcationic surfactant is formed on the surface of the colloid may be usedas the sacrificial template although the sacrificial colloid is apolymer soluble in the solvent.

It is possible to manufacture a metal oxide precursor/polymer compositenanofiber which has a rugged structure and in which the sacrificialtemplate and the nanoparticle catalyst in the hollow structure of theapoferritin are uniformly distributed by dispersing the nanoparticlecatalyst synthesized using the apoferritin described above and thespherical sacrificial template in the electrospinning solution and usingthe electrospinning method. Mesopores and macropores are formed usingthe decomposition of the sacrificial polymer and the crystallization anddiffusion of the metal oxide which sequentially occur during thehigh-temperature heat treatment of the composite nanofiber thus formed,and a one-dimensional porous nanotube on which the nanoparticlecatalysts are uniformly loaded can be synthesized through the diffusionof the nanoparticle catalyst occurring at the time of tube formation. Inthe case of the one-dimensional porous nanotube which have mesopores andmacropores and contains the nanoparticle catalyst, the diameter of thenanotube structure is in a diameter range of from 50 nm to 5 μm (theouter diameter may be in a size range of from 50 nm to 2 μm, and theinner diameter may be in a size range of from 40 nm to 1.95 μm), thethickness between the inner wall and the outer wall (thickness of theshell) is in a range of from 10 nm to 50 nm, and the length is in arange of from 1 μ to 100 μm.

The one-dimensional porous nanotube which constitutes the nano-structureand has a metal oxide semiconductor double pore distribution is notparticularly limited to a specific material as long as the value of theelectrical resistance and the electrical conductivity is changed by theadsorption and desorption of gas. Specifically, the one-dimensionalporous nanotube may be a one-dimensional porous nanotube which has adouble pore distribution and is composed of one or a composite materialof two or more selected from ZnO, SnO₂, WO₃, Fe₂O₃, Fe₃O₄, NiO, TiO₂,CuO, In₂O₃, Zn₂SnO₄, Co₃O₄, PdO, LaCoO₃, NiCo₂O₄, Ca₂Mn₃O₈, V₂O₅, Cr₂O₃,Nd₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃, Tb₄O₇, Dy₂O₃, Ho₂O₃, Er₂O₃, Yb₂O₃, Lu₂O₃,Ag₂V₄O₁₁, Ag₂O, Li_(0.3)La_(0.57)TiO₃, LiV₃O₈, InTaO₄, CaCu₃Ti₄O₁₂,Ag₃PO₄, BaTiO₃, NiTiO₃, SrTiO₃, Sr₂Nb₂O₇, Sr₂Ta₂O₇, orBaSr_(0.5)Co_(0.8)Fe_(0.2)O₃₋₇.

By using a gas sensor member suing the one-dimensional porous metaloxide nanotube 100 which is fabricated above, contains the nanoparticlecatalyst 110, and has the mesopores 121, macropores 131, and with adouble pore distribution, it is possible to manufacture anultrasensitive/highly selective sensor which can diagnose a human bodydisease at the early stage by selectively sensing a specific biomarkergas which acts as a biomarker in the exhaled breath of a human body andcan also be applied to an environmental sensor capable of monitoringharmful environmental gases in real time. In particular, an optimalstructure in which the sensing materials in all areas can effectivelyresponse to the gas is formed by forming pores on the nanotube surfaceto maximize the gas flow toward the sensing material. In addition, theporous tube structure thus fabricated has a thin surface and anincreased surface area so as to have a great advantage of being able tomaximize the sensing characteristics of the sensing material with asmall amount of catalyst and an advantage of being able to manufacturevarious kinds of gas sensor members not only easily and rapidly but alsoin a large scale.

FIG. 2 is a flow chart of a method of manufacturing a gas sensor memberusing a one-dimensional porous metal oxide semiconductor nanotube whichis synthesized using the electrospinning method, contains a nanoparticlecatalyst, and has a double pore distribution including a number of poresaccording to an embodiment of the inventive concept. According to theflow chart in FIG. 2, the method for manufacturing a gas sensor memberincludes a step S210 of synthesizing a nanoparticle catalyst using anapoferritin having a hollow structure, a step S220 of preparing anelectrospinning solution by mixing the nanoparticle catalyst thussynthesized and the spherical sacrificial template with a metal oxideprecursor/polymer electrospinning solution and stirring them together, astep S230 of synthesizing a metal oxide precursor/polymer compositenanofiber in which the spherical sacrificial template and thenanoparticle catalyst are uniformly distributed through electrospinning,a step S240 of forming macropores (50 nm to 300 nm) through thedecomposition of the spherical polymer sacrificial template by ahigh-temperature heat treatment and mesopores (0.1 nm to 50 nm)utilizing the polymer decomposition and the metal oxide diffusion whichsequentially occur, and a step S250 of synthesizing a metal oxidenanotube that is uniformly functionalized with the nanoparticle catalystand has mesopores and macropores through the continuous high-temperatureheat treatment. The respective steps will be described below in moredetail.

First, the step S210 of synthesizing a nanoparticle catalyst using anapoferritin is described. The apoferritin used in this step S210includes ferritin extracted from equine spleen, and an apoferritinprepared by removing the iron ion present inside a ferritin obtainedfrom liver, spleen, or the like of a human body or a swine regardless ofthe extraction site may be used. For storage of the apoferritin,solutions of sodium chloride (NaCl) at various concentrations includingsaline can be used as a solution to keep the apoferritin, and theapoferritin is required to be stored under refrigeration at 4° C. orlower. In addition, in order to embed the metal salt in the apoferritin,a an acidic solution having a pH of from 2 to 3 or a basic solutionhaving a pH in a range of from 7.5 to 8.5 (or in a pH range of from 7.5to 9) is preferable, the apoferritin is immersed in a solutioncontaining a metal salt dissolved therein for about from 1 hour to 24hour so that the metal salt can be sufficiently diffused into theapoferritin. The concentration of the solution for storage, such assaline containing the apoferritin is set to be in a range of from 0.1 to200 mg/ml. As the solvent used when preparing the metal salt solution, acommercially available solvent such as ethanol, water, chloroform,N,N′-dimethylformamide, dimethylsulfoxide, N,N′-dimethylacetamide, orN-methylpyrrolidone can be used, and the solvent is not limited to aparticular solvent as long as it dissolves a metal salt. Examples of thenanoparticle catalyst embedded in the apoferritin may include Pt, Pd,Rh, Ru, Ni, Co, Cr, Ir, Au, Ag, Zn, W, Sn, Sr, In, Pb, Fe, Cu, V, Ta,Sb, Sc, Ti, Mn, Ga, or Ge, the nanoparticle catalyst composed of one ortwo or more of these may be synthesized in an alloy form. It is possibleto use one heterogeneous nanoparticle catalyst selected form the groupconsisting of a metal-metal, a metal-metal oxide, or a metal oxide-metaloxide as the alloy nanoparticle catalyst. Examples of the representativemetal-metal oxide nanoparticle catalyst may include Pt/IrO₂, Pt/RuO₂,Pt/Rh₂O₃, Pt/NiO, Pt/Co₃O₄, Pt/CuO, Pt/Ag₂O, Pt/Fe₂O₃, Au/IrO₂, Au/RuO₂,Au/Rh₂O₃, Au/NiO, Au/Co₃O₄, Au/CuO, and Au/Ag₂O, examples of themetal-metal nanoparticle catalyst may include Pt—Au, and the metaloxide-metal oxide catalyst may be a metal oxide-metal oxide catalystcomposed of two kinds selected from TiO₂, ZnO, WO₃, SnO₂, IrO₂, In₂O₃,V₂O₃, and MoO₃ of n-type metal oxides and Ag₂O, PdO, RuO₂, Rh₂O₃, NiO,Co₃O₄, CuO, Fe₂O₃, Fe₃O₄, V₂O₅, and Cr₂O₃ of p-type metal oxides. As thereducing agent to reduce the metal salt contained inside the hollowstructure of the apoferritin, a generally used reducing agent such assodium borohydride (NaBH₄), formic acid (HCOOH), oxalic acid (C₂H₂O₄) orlithium aluminum hydride (LiAIH₄) may be used, and a reducing agentcapable of reducing the metal salt so as to form a metallic nanoparticlecatalyst may be used without any particular limitation. The solutionsubjected to the reduction of the metal salt inside the apoferritin by areducing agent is then subjected to the centrifugation to separate theapoferritin protein embedding the nanoparticle catalyst therefrom, andthe rotational speed of the centrifugal separator used at this time ispreferably from 10,000 rpm to 13,000 rpm.

Next, the step S220 of preparing a metal oxide precursor/polymer mixedelectrospinning solution which contains the nanoparticle catalyst thatis synthesized and embedded inside the hollow structure of theapoferritin and a spherical sacrificial template is described. In thisstep S220, the nanoparticle catalyst that is synthesized and embeddedinside the hollow structure of the apoferritin and sacrificial templatecolloids are added to a metal oxide precursor/polymer mixedelectrospinning solution, and the mixture stirred such that thenanoparticle catalyst and the sacrificial template colloids areuniformly dispersed in the electrospinning solution, whereby the mixedelectrospinning solution is prepared. As the solvent used when preparingthe electrospinning solution, a commercially available solvent such asN,N′-dimethylformamide, dimethylsulfoxide, N,N′-dimethylacetamide,N-methylpyrrolidone, deionized (DI) water, or ethanol may be used, butit is required to select a solvent which can dissolve the metal oxideprecursor and the polymer at the same time. In addition, the polymer andthe sacrificial template used herein are not limited to specificsubstances as long as they are a substance that is removed through thehigh-temperature heat treatment, and representative examples thereof mayinclude polymethyl methacrylate (PMMA), polyvinylpyrrolidone (PVP),polyvinyl acetate (PVAc), polyvinyl alcohol (PVA), polyacrylonitrile(PAN), polyethylene oxide (PEO), polypropylene oxide (PPO), polyethyleneoxide copolymer, polypropylene oxide copolymer, polycarbonate (PC),polyvinylchloride (PVC), polycaprolactone, and polyvinylidene fluoride.

In addition, the metal oxide precursor used in this step is required tobe dissolved in a solvent, the metal oxide precursor is not limited to aspecific metal salt as long as it is a precursor containing a metal saltcapable of forming a metal oxide semiconductor nanofiber or nanotubewhich exhibits a change in resistance by the adsorption and desorptionof gas through the high-temperature heat treatment, such as SnO₂, WO₃,CuO, NiO, ZnO, Zn₂SnO₄, Co₃O₄, Cr₂O₃, LaCoO₃, V₂O₅, IrO₂, TiO₂, Er₂O₃,Tb₂O₃, Lu₂O₃, Ag₂O, SrTiO₃, Sr₂Ta₂O₇, BaTiO₃, orBa_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O₃₋₇.

The weight ratio of the polymer to the metal oxide precursor for formingthe electrospinning solution is preferably about from 1:1 to 2, and theratio to the polymer to the nanoparticle catalyst synthesized using theapoferritin is preferably about from 1:0.00001 to 0.1 and may be in arange of from 0.000001 to 1. In addition, the weight ratio of thespherical sacrificial template to the polymer used in the step S220 ispreferably about from 1:1 to 2 and may be in a range of from 0 wt % to50 wt %. In addition, the weight ratio of the metallic nanoparticlecatalyst encapsulated in the protein may be in a range of from 0.001 wt% to 50 wt % with respect to the metal oxide precursor constituting themetal oxide precursor/polymer composite nanofiber. The size of thespherical sacrifice template having a size in a range of from 50 nm to 1μm is preferably set in consideration of the size of the pores to beformed, and the kind of the metal salt in the apoferritin is selected inconsideration of the selectivity of the gas to be sensed, whereby it ispossible to manufacture a gas sensor member having variouscharacteristics.

As the procedure to prepare a mixed electrospinning solution in the stepS220, the metal oxide precursor is first dissolved in a solvent, and theapoferritin encapsulating the nanoparticle catalyst synthesized inadvance and the spherical sacrificial template are sequentiallydispersed in the solution. Here, as the dispersing method, there is amethod to stir for 1 hour or longer at a rotational speed of 500 rpm. Inorder to impart a viscosity to the solution thus prepared to a certainextent to which the electrospinning is facilitated, a polymer is addedto the solution at an appropriate ratio and the mixture is sufficientlystirred until the polymer is completely dissolved in the solution. Asthe stirring condition, the mixture is preferably stirred at from roomtemperature to 50° C., and it is sufficiently stirred for from 5 hoursto about 48 hours so that the apoferritin encapsulating the nanoparticlecatalyst and the sacrificial template colloids are uniformly mixed inthe metal oxide precursor and polymer solution.

The step S230 is carried out by electrospinning the electrospinningmixed solution prepared above to form a rugged metal oxideprecursor/polymer composite nanofiber in which the spherical sacrificialtemplate and the nanoparticle catalyst in the apoferritin are uniformlydistributed.

Upon conducting the electrospinning in order to carry out the step S230,the metal oxide precursor/polymer mixed electrospinning solutioncontaining the nanoparticle catalyst and the spherical sacrificialtemplate thus prepared is filled in a syringe, the syringe is pressedusing the syringe pump at a constant rate so that a certain amount ofthe electrospinning solution is discharged therefrom. Theelectrospinning system may be constituted by a high voltage apparatus, agrounded conductive substrate, a syringe, and a syringe nozzle, a highvoltage about from 5 kV to 30 kV is applied to between the solutionfilled in the syringe and the conductive substrate to generate theelectric field, and the electrospinning solution discharged through thesyringe nozzle is ejected in a long nanofiber form due to the electricfield thus generated, whereby the electrospinning is conducted. Theelectrospinning solution ejected in a long nanofiber form is obtained asa solid polymer fiber as the solvent contained in the electrospinningsolution is evaporated and volatilized and a composite fiber containingthe metal oxide precursor, the nanoparticle catalyst encapsulated in theapoferritin, and the spherical sacrificial template is obtained at thesame time. The discharge speed may be controlled to from 0.01 ml/min to0.5 ml/min, and it is possible to fabricate a metal oxideprecursor/polymer/nanoparticle catalyst composite nanofiber having adesired diameter and a rugged structure by controlling the voltage andthe discharge rate.

Through the step S240, it is possible to form a metal oxide nanotubestructure through the high-temperature heat treatment of the compositenanofiber thus fabricated and to manufacture a one-dimensional porousmetal oxide nanotube in which mesopores and macropores are distributedon the surface of the metal oxide nanotube and the nanoparticlecatalysts are uniformly distributed on such a nanotube structure at thesame time. Through the high-temperature heat treatment in a temperaturerange of from 500 to 800° C., the spherical polymer substance used asthe sacrificial template and the apoferritin are all removed to formmacropores (50 nm to 300 nm) and mesopores (0.1 nm to 50 nm), and themacropores are partially filled by the crystallization and diffusion ofthe metal oxide taking place after the decomposition of the polymer toform a plurality of mesopores (0.1 nm to 50 nm) on the nanotube surface.

Additionally, through the step S250, the nanoparticle catalystencapsulated in the apoferritin is uniformly loaded on the inner andouter walls and in the inside of the porous nanotube during the heattreatment as the apoferritin is removed. The structure that is finallyformed through the step S250 is a one-dimensional porous metal oxidenanotube structure which has a plurality of mesopores and macropores andin which the nanoparticle catalyst is uniformly loaded on the inner andouter walls and in the inside of the tube.

FIG. 3 is a diagram schematically illustrating the manufacturing processaccording to the method for manufacturing a gas sensor member using aone-dimensional porous metal oxide nanotube which is fabricated using anelectrospinning method, contains a nanoparticle catalyst, and has adouble pore distribution according to an embodiment of the inventiveconcept.

The step S310 of the first step is a process showing the intermediatestep of the high-temperature heat treatment of the metal oxideprecursor/polymer composite nanofiber on which the spherical sacrificialtemplate and the nanoparticle catalyst encapsulated in the apoferritinare uniformly loaded, and it is a step showing the intermediate step inwhich pores having various sizes are formed as the spherical sacrificialtemplate and the apoferritin are removed and a metal oxide tube isformed through the Ostwald ripening at the same time.

In the step S320 of the second step, as the spherical sacrificialtemplate, the apoferritin, and the polymer matrix in the composite fiberare all removed after the final high-temperature heat treatment, thecrystallization and diffusion of the metal oxide occurs, and finally,there are a number of mesoporous and macropores on the inner and outerwalls of the nanotube, and the nanoparticle catalysts are diffused sothat a one-dimensional porous metal oxide nanotube which has a doublepore distribution and in which the nanoparticle catalysts are uniformlyloaded on the outer surface and in the inside of the nanotube issynthesized.

FIG. 4 is a diagram illustrating the mechanism that a plurality ofmesopores and macropores are formed on the nanotube surface during thehigh-temperature heat treatment. Specifically, the macropores are formedas the sacrificial polystyrene template is decomposed during thehigh-temperature heat treatment and the mesopores are then formed as themacropores are filled by the crystallization and diffusion of the metaloxide.

FIG. 5 is a diagram illustrating the procedure that the proteintemplates in a 12 nm size are densely gathered in between thepolystyrene templates distributed on the nanofibers and the denselygathered protein templates are decomposed during the high-temperatureheat treatment to contribute to the formation of the mesopores. Here,the highly dispersible protein templates can be densely gathered in anarrow space in between the polystyrene templates.

As described above, in the method for manufacturing a gas sensor memberusing a one-dimensional porous metal oxide nanotube which is fabricatedusing a sacrificial template, an electrospinning method, and the timedifference between the decomposition of a polymer and the diffusion of ametal oxide, contains a nanoparticle catalyst, and have a number ofmesopores and macropores according to embodiments of the inventiveconcept, it is possible to improve the gas sensing effect by forming aone-dimensional nanotube structure having a large surface area requiredfor the reaction with a gas and forming pores at the same time tomaximize the gas flow toward the sensing material and to greatly improvethe reaction rate characteristic of the gas sensor, the sensitivitycharacteristics, and the selectivity by loading a catalyst which isuniformly distributed using the characteristics of a protein and has achemical/electronic sensitization effect unlike the existing catalyst.

Hereinafter, the inventive concept will be described in detail withreference to Embodiment Examples and Comparative Examples. EmbodimentExamples and Comparative Examples are only for explaining the inventiveconcept, and the inventive concept is not limited to the followingEmbodiment Examples.

Embodiment Example 1: Preparation of Pt Nanoparticle Catalyst UsingApoferritin as Template

The following procedure is used in order to synthesize a Pt nanoparticlecatalyst of 3 nm or less inside an apoferritin having a hollowstructure.

The apoferritin solution (Sigma Aldrich) is dispersed in a 0.15 M NaClaqueous solution at a concentration of 35 mg/ml. A basic solution suchas NaOH is added to the apoferritin solution as described above toadjust the pH to about 8.5, thereby preparing an optimum condition forthe diffusion of Pt metal ion diffuse into the apoferritin. The basicsolution used herein is not limited as long as it is a basic aqueoussolution in addition to NaOH. The precursor of Pt metal ion that isencapsulated in the apoferritin is H₂PtCl₆.H₂O, and it is prepared inthe form of an aqueous solution by dissolving 16 mg of H₂PtCl₆.H₂O in 1g of DI water. The aqueous solution of metal salt precursor thusprepared is gradually dropped into the apoferritin solution having anadjusted pH drop by drop using a dropper and mixed. The mixed solutionis stirred for 1 hour so that the Pt metal ions diffuse into the insideof the apoferritin having a hollow structure. The stirring conditionsdescribed above means that the stirring is conducted at room temperatureand a rotational speed of 100 rpm for 1 hour. After the mixed solutionis sufficiently stirred, the metal ions in the apoferritin are reducedusing a reducing agent so as to synthesize a Pt nanoparticle catalyst inthe inside of the apoferritin. As the reducing agent used herein, aNaBH₄ aqueous solution is representative. The reducing agent, NaBH₄ usedat this time is prepared in the form of an aqueous solution at aconcentration of 40 mM and added by 0.5 ml.

The aqueous solution in which the Pt nanoparticle catalyst that isencapsulated in the hollow structure of the apoferritin and synthesizedby the method as described above is dispersed contains the reducingagent and the ligand of the metal salt, and thus the synthesized Ptnanoparticle catalyst is required to be extracted throughcentrifugation. At this time, as the centrifugal condition, it ispreferable to conduct the centrifugation at about 12,000 rpm for 10minutes or longer. The Pt nanoparticle catalyst encapsulated in theapoferritin extracted by centrifugation is dispersed in DI water,thereby preparing an aqueous solution in which the Pt nanoparticlecatalyst is dispersed in the inside of the apoferritin.

FIGS. 7A and 7B are transmission electron microscope (TEM) images of theapoferritin encapsulating the Pt nanoparticle catalyst synthesized bythe procedure described above, FIG. 7C illustrates the surface charge,and FIG. 7D illustrates the size distribution. From the images takenusing a transmission electron microscope, it can be seen that the Ptnanoparticle catalyst is favorably dispersed, and this is due to thedispersion effect caused by the repulsive force between the positively(+) charged protein shells. In addition, the nanoparticle catalyst has asize distribution in from 1 to 3 nm.

Embodiment Example 2: Fabrication of One-Dimensional Porous Tin Oxide(SnO₂) Nanotube 100 Structure having Pt Nanoparticle Catalyst UniformlyLoaded on Inner and Outer Walls of Tube an having Mesopores andMacropores

First, 0.25 g of tin chloride dihydrate of the metal oxide precursor isadded to a mixed solvent of 1.35 g of DMF and 1.35 g of ethanol anddissolved at room temperature. Next, 0.3 g of spherical polystyrene(diameter: 200 nm) colloids to serve as the sacrificial template asillustrated in FIG. 6 is added to and sufficiently dispersed in thesolution in which the metal salt precursor is dissolved. The polystyrenecolloids used in Embodiment Example 2 have an anionic surfactant formedon the surface, and thus it exhibits excellent dispersibility, isinsoluble in DMF of the solvent, and is removed during the heattreatment to be conducted later to contribute to the formation ofmacropores. In Embodiment Example 2, polystyrene polymer beads having asize of 200 nm are used as the colloidal template, but the kind of thepolymer template is not particularly limited. Specifically, it may beone kind or a mixture of two or more kinds selected from polymethylmethacrylate (PMMA), polyvinylpyrrolidone (PVP), polyvinyl acetate(PVAc), polyvinyl alcohol (PVA), polystyrene (PS), polyacrylonitrile(PAN), polyvinylidene fluoride (PVDF), polyacrylic acid (PAA),polydiallydimethyμmmonium chloride (PDADMAC), or polystyrene sulfonate(PSS). In addition, the sacrificial template has a size in the range offrom 50 nm to 1 μm, it is dispersed without being decomposed when mixedwith an electrospinning solution, and a polymer colloid that isinsoluble in a solvent since a charged ion or a charged anionic orcationic surfactant is formed on the surface of the colloid may be usedas the sacrificial template although the sacrificial colloid is apolymer soluble in the solvent. The dispersion condition of thecolloidal polystyrene template means that the stirring is conducted at arotational speed of 500 rpm for about 10 minutes, and the diameter ofthe polystyrene used in the above is not limited to 200 nm a colloidalsolution of polystyrene having various diameters may be used. Inaddition, 200 mg of the aqueous solution of the Pt nanoparticle catalystsynthesized in Embodiment Example 1 is added to the mixed solution(polystyrene colloid+metal salt+mixed solvent) and mixed. In order toincrease the viscosity of the solution in which spherical polystyrenepolymer and the nanoparticle catalyst encapsulated in the apoferritinare uniformly mixed thus synthesized, 0.35 g of polyvinylpyrrolidone(PVP) polymer having a molecular weight of 1,300,000 g/mol is addedthereto, and the mixture is stirred at room temperature and a rotationalspeed of 500 rpm for 24 hours to prepare an electrospinning solution.The electrospinning solution thus prepared is filled in the syringe(Henke-Sass Wolf, 10 mL NORM-JECT®), the syringe is connected to thesyringe pump, the electrospinning solution is pushed out at a dischargespeed of 0.1 ml/min, and voltage between the nozzle (needle, 25 gauge)used during electrospinning and the collector gathering the nanofibersis set to 14 kV, thereby conducting the electrospinning. At this time,stainless steel plate is used as the nanofiber collecting plate, and thedistance between the nozzle and the collector is set to 26 cm.

FIG. 8 is a scanning electron microscope (SEM) image of the nanofiberwhich is obtained by electrospinning and contains the metal oxideprecursor, polyvinylpyrrolidone polymer, the spherical polystyrenesacrificial template, and the Pt nanoparticle catalyst encapsulated inthe hollow structure of the apoferritin. It can be seen that aone-dimensional nanofiber is formed, and it is confirmed that thenanofiber has a rugged structure since it contains sphericalpolystyrene. The diameter of the nanofiber thus synthesized has a valuebetween 200 nm and 300 nm.

The nanofiber which is synthesized by the method as described above andcontains the metal oxide precursor, polyvinylpyrrolidone polymer, thespherical polystyrene sacrificial template, and the Pt nanoparticlecatalyst encapsulated in the hollow structure of the apoferritin ismaintained at 600° C. for 1 hour and then cooled to room temperature ata cooling rate of 40° C./min. The heat treatment is conducted in an airatmosphere using the small electric furnace Vulcan 3-550 manufactured byNey. The apoferritin protein encapsulating the nanoparticle catalyst andthe polymer are all decomposed through the heat treatment. In addition,the heat treatment is conducted in the air atmosphere, thus the metalsalt precursor on the nanofiber surface is first oxidized into metaloxide particles through the nucleation and particle growth, the metalsalt precursor inside the nanofiber is also oxidized through the Ostwaldripening and diffused toward the nanofiber surface to form a nanotube,at the same time, the polystyrene template is removed by the heattreatment, and the macropores are partially filled by the diffusion ofmetal oxide to form the mesopores and macropores on the surface of thenanotube. In addition, the Pt nanoparticle catalyst also has asignificantly small size so as to diffuse toward the nanotube surfacetogether with the tin oxide particles and to be loaded on the inner andouter walls of the tin oxide nanotube. As a result, a one-dimensionalporous nanotube structure in which a number of pores are distributed onthe surface of the tin oxide nanotube structure and the Pt nanoparticlecatalysts are uniformly distributed.

FIGS. 9A and 9B are SEM images of the one-dimensional porous tin oxidenanotube which contains the Pt nanoparticle catalyst synthesized inEmbodiment Example 1 and has a double surface pore distribution. Thediameter of the nanotube thus fabricated has a size of about from 50 nmto 5 μm, the thickness between the outer wall and the inner wall of thetube is in a range of from 10 nm to 50 nm. In addition, the size of themesopores formed on the nanotube surface is in a range of from 0.1 nm to50 nm, and the macropores has a size of from 50 nm to 300 nm.

FIGS. 10A, 10B, and 100 are TEM images of the one-dimensional porous tinoxide nanotube which contains the Pt nanoparticle catalyst synthesizedin Embodiment Example 1 and has a double surface pore distribution. Itcan be seen that the Pt nanoparticle catalyst is inside theone-dimensional porous tin oxide nanotube by the transmission electronmicroscopy at a high magnification, and it is confirmed that the Ptnanoparticle catalyst exhibits crystallinity in the one-dimensionalporous tin oxide nanotube by the selected area electron diffraction(SAED) pattern illustrated in FIG. 10D. In addition, it is confirmedthat various pores having a size of from 5 to 150 nm are distributed onthe tin oxide nanotube surface from the images taken using atransmission electron microscope. In addition, it is confirmed that thePt nanoparticle catalyst is uniformly distributed in the tin oxidenanotube structure from the energy dispersive X-ray spectrometer (EDS)images through the TEM illustrated in FIG. 10E.

FIGS. 11A and 11B are the thermogravimetric analysis graph and thephotoelectron spectroscopic (XPS) analysis graph of the one-dimensionalporous tin oxide nanotube which contains the Pt nanoparticle catalystsynthesized and has mesopores and macropores, respectively. Through thethermogravimetric analysis (TGA), it can be seen that the macropores (50to 300 nm) are formed as the sacrificial template polymer is removed atabout 300° C. and the mesopore (0.1 to 50 nm) are formed as thecrystallization and diffusion of the metal oxide subsequently occur tocover the macropores at about 400° C. Through the photoelectronspectroscopic (XPS) analysis, it is confirmed that PtO that is anoxidized state of the Pt nanoparticles and the Pt metallic form coexist.

Comparative Example 1: Fabrication of Pure Tin Oxide Nanotube NotContain Nanoparticle Catalyst

In Comparative Example 1 to be compared with Embodiment Example 2, apure tin oxide nanotube which does not contain the Pt nanoparticlecatalyst and does not have circular or elliptical pores is synthesized.Specifically, 0.25 g of tin chloride dihydrate of the metal oxideprecursor is dissolved in a mixed solvent (1.35 g of DMF+1.35 g ofethanol), and in order to increase the viscosity of the mixed solution,0.35 g of polyvinylpyrrolidone (PVP) polymer having a molecular weightof 1,300,000 g/mol is added thereto and the mixture is sufficientlystirred. The stirring conditions referred to herein means that thestirring is conducted at a rotational speed of 500 rpm for at least 5hours. The tin oxide precursor/polymer mixed electrospinning solutionthus prepared is filled in the syringe for electrospinning (Henke-SassWolf, 10 mL NORM-JECT®), the syringe is connected to the syringe pump,and the electrospinning solution is pushed out at a discharge speed of0.1 ml/min, thereby conducting the electrospinning. The needle usedduring electrospinning is a 25 gauge needle, and a high voltage of about14 kV is applied while maintaining the distance between the nozzle thecollector to collect the nanofibers to be 26 cm, thereby fabricating thetin oxide precursor/polymer composite nanofiber.

The tin oxide precursor/polymer composite nanofiber synthesized above issubjected to a high-temperature heat treatment to remove the polymer,and the tin oxide precursor is subjected to the oxidation to form tinoxide. The high-temperature heat treatment is conducted for 1 hour at600° C., the heating rate is constantly maintained at 10° C./min, andthe cooling rate is constantly maintained at 40° C./min. Here, it isimportant to set the heating rate to 10° C./min to be relatively highfor the formation of the nanotube structure.

FIG. 12 is a SEM image of the pure tin oxide nanotube fabricated inComparative Example 1. It is confirmed that the diameter of the tinoxide nanotube thus synthesized is in a range of from 50 nm to 5 μm, andthe thickness between the inner and outer walls of the nanotube has avalue between 10 and 50 nm.

Comparative Example 2: Fabrication of Pure Tin Oxide One-DimensionalPorous Nanotube Not Containing Nanoparticle Catalyst

In Comparative Example 2 to be compared with Embodiment Example 2, apure tin oxide one-dimensional porous nanotube having circular andelliptical pores is synthesized by adding a spherical polystyrenesacrificial template but not adding a Pt nanoparticle catalyst embeddedin an apoferritin. Specifically, 0.25 g of tin chloride dihydrate of themetal oxide precursor is dissolved in a mixed solvent (1.35 g ofDMF+1.35 g of ethanol). Additionally, 0.3 g of polystyrene colloidswhich has a diameter of 200 nm and serves as the spherical sacrificialtemplate is added thereto and dispersed. The dispersion conditionreferred to herein means that the stirring is conducted at a rotationalspeed of 500 rpm for at least 1 hour. In order to increase the viscosityof the tin oxide/polystyrene composite solution, 0.35 g ofpolyvinylpyrrolidone (PVP) polymer having an average weight of 1,300,000g/mol is added thereto and the mixture is sufficiently stirred. Thestirring conditions referred to herein means that the stirring isconducted at a rotational speed of 500 rpm for at least 10 hours. Themetal precursor/polystyrene sacrificial template/polymer electrospinningsolution that is sufficiently stirred is filled in the syringe forelectrospinning (Henke-Sass Wolf, 10 mL NORM-JECT®), the syringe isconnected to the syringe pump, the electrospinning solution is pushedout at a discharge speed of 0.1 ml/min, the needle used duringelectrospinning is a 25 gauge needle, and a high voltage of about 14 kVis applied while maintaining the distance between the nozzle thecollector to collect the nanofibers to be 26 cm, thereby fabricating thetin oxide precursor/polystyrene sacrificial template/polymer compositenanofiber.

The tin oxide precursor/polystyrene sacrificial template/polymercomposite nanofiber synthesized above is subjected to a high-temperatureheat treatment to remove the polymer, circular or elliptical macroporesare formed as the spherical polystyrene sacrificial template isdecomposed, and mesopores are then formed as the macropores are filledby the crystallization and diffusion of tin oxide, thereby forming aone-dimensional porous pure tin oxide nanotube. The high-temperatureheat treatment is conducted for 1 hour at 600° C.

FIGS. 13A and 13B are SEM images of the pure tin oxide nanotubestructure which is fabricated in Comparative Example 2 and has circularor elliptical pores. It is confirmed that the one-dimensional porous tinoxide nanotube thus fabricated has a diameter in a range of from 50 nmto 5 μm, and the thickness between the inner and outer walls of thenanotube has a value in a range of from 10 and 50 nm. Here, the size ofthe mesopores has a value between 0.1 nm and 50 nm, and the macroporeshas a size of from 50 nm to 300 nm. It is confirmed that the size of thepores is relatively large unlike in Embodiment Example 2 since theparticle growth of tin oxide is not interfered by the protein template,apoferritin.

Experimental Example 1: Manufacture of gas sensor using one-dimensionalporous tin oxide nanotube having Pt nanoparticle catalyst uniformlyloaded on inner and outer walls of tube and a number of circular andelliptical pores, tin oxide one-dimensional porous nanotube havingpores, and pure tin oxide nanotube, and evaluation on characteristicsthereof.

In order to manufacture exhaled breath sensors using the sensingmaterials for gas sensor fabricated in Embodiment Examples 1 and 2 andComparative Examples 1 and 2, 6 mg of each of the one-dimensional poroustin oxide nanotube which contains the Pt nanoparticle catalyst and has anumber of mesopores and macropores, the one-dimensional porous tin oxidenanotube, and tin oxide nanotube is dispersed in 100 μl of ethanol andsubjected to the ultrasonic cleaning for 1 hour to be ground. During thegrinding, the porous nanotube structure synthesized above may have aporous nanotube structure shortened in the longitudinal direction.

The one-dimensional porous tin oxide nanotube uniformly loaded with thePt nanoparticle catalyst and having a number of circular and ellipticalpores, the one-dimensional porous tin oxide nanotube, and the tin oxidenanotube are coated on the upper portion of the alumina substrate whichhas a size of 3 mm'3 mm and on which parallel gold (Au) electrodes areformed at an interval of 150 μm by drop coating. As the coating process,3 μl of a mixed solution of the one-dimensional porous nanotube loadedwith the Pt nanoparticle catalyst, the one-dimensional porous tin oxidenanotube, and the tin oxide nanotube dispersed in ethanol was coated onthe alumina substrate having the sensor electrode using a micropipetteand dried on a hot plate at 60° C., and this process was repeated 4 to 6times so as to coat a sufficient amount of sensing material on the upperportion of the alumina sensing plate.

In addition, for the simulation characteristic evaluation as a exhaledbreath sensor, the response characteristics of the gas sensor thusfabricated with respect to acetone (CH₃COCH₃), hydrogen sulfide (H₂S),and toluene (C₆H₅CH₃) of the biomarker gas for the diagnosis ofdiabetes, halitosis, and lung cancer, respectively, were evaluated at arelative humidity of 85-95% RH to be similar to the humidity of gases inthe exhaled breath of a human body by changing the concentration of therespective gases from 5 to 4, 3, 2, 1, 0.6, 0.4, 0.2, and 0.1 ppm andmaintaining the driving temperature of the sensor at 350° C. at the sametime. In addition, in Experimental Example 1, the selective gas sensingcharacteristics were investigated by evaluating the sensingcharacteristics not only for acetone (CH₃COCH₃), hydrogen sulfide (H₂S),and toluene (C₆H₅CH₃) of the representative examples of the volatileorganic compound gas but also for nitric oxide (NO), carbon monoxide(CO), ammonia (NH₃), and pentane (C₅H₁₂) of the biomarker gas of asthma,chronic obstructive pulmonary disease, nephritis, and heart disease.

Additionally, in order to evaluate the capability to sense the exhaledbreath of a healthy person and a real diabetic patient, the exhaledbreath of 10 healthy people was prepared and the exhaled breath of asimulated diabetic patient was prepared so as to be similar to theexhaled breath of a real diabetic patient. The exhaled breath thusprepared was directly sensed by the sensor array, and the measuredsensing results were subjected to the principal component analysis (PCA)to compare the exhaled breath of the diabetic patient with that ofhealthy people.

FIG. 14A is a graph illustrating the time course response properties(R_(air)/R_(gas), where R_(air) means the resistance value of the metaloxide material when the air is injected, and R_(gas) means theresistance value of the metal oxide material when acetone is injected)when the concentration of acetone decreases from 5 to 4, 3, 2, 1, 0.6,0.4, 0.2, and 0.1 ppm at 350° C. In addition, FIG. 14B is a graphillustrating the detection limit of the one-dimensional porous nanotubeloaded with the Pt nanoparticle catalyst using linear approximation.

As illustrated in FIG. 14, the sensing characteristics of theone-dimensional porous tin oxide nanotube sensing material on which thePt nanoparticle catalyst embedded in the hollow structure of aapoferritin is loaded through the heat treatment with respect to 5 ppmacetone is 21.1 times as high as that of the one-dimensional porous tinoxide nanotube not containing a catalyst and 38 times as high as that ofthe pure tin oxide nanotube. In addition, the detection limit of theone-dimensional porous tin oxide nanotube sensing material loaded withthe Pt nanoparticle catalyst that is obtained based on the sensingresults measured at the acetone concentration of 5, 4, 3, 2, 1, 0.6,0.4, 0.2, and 0.1 ppm using the linear approximation is 2.1 as thesensitivity (R_(air)/R_(gas)) at the acetone concentration of 10 ppb.

FIG. 15 is a graph illustrating the response value of a gas sensor usingthe one-dimensional porous tin oxide nanotube on which the Ptnanoparticle catalyst embedded in the apoferritin is loaded through theheat treatment and has a number of circular and elliptical pores withrespect to hydrogen sulfide, toluene, nitrogen monoxide, carbonmonoxide, ammonia, and pentane of biomarker gases of other diseasestogether with the response value with respect to acetone of thebiomarker gas of diabetes and lipolysis at a concentration of 1 ppm and350° C.

As illustrated in FIG. 15, a gas sensor fabricated using theone-dimensional porous tin oxide nanotube which has a double poredistribution and is loaded with the Pt nanoparticle catalyst exhibitsspecifically excellent selective sensing characteristics with respect toacetone of the biomarker gas of diabetes and lipolysis compared tohydrogen sulfide, toluene, pentane, carbon monoxide, ammonia, andnitrogen monoxide of biomarker gases of other diseases.

FIG. 16 is a diagram illustrating a process to prepare the exhaledbreath of 10 simulated diabetic patients by collecting the exhaledbreath of 10 healthy people in Tedler bags and quantitatively injectingconcentrated acetone so as to have an acetone concentration of about 2ppm in the exhaled breath of a human body. As illustrated in FIG. 16, itis possible to set the acetone concentration in the exhaled breath of ahuman body to 2 ppm by using a diaphragm pump to quantitatively suck anddischarge the gas.

FIG. 17 is a graph illustrating the results of principal componentanalysis (PCA) on the sensing results obtained by injecting the exhaledbreath of 10 healthy people actually collected and the exhaled breath ofthe simulated diabetic patients into the sensor array composed of theporous tin oxide nanotube which is loaded with a platinum nanoparticlecatalyst and have mesopores and macropores, the tin oxide nanotubeloaded with a platinum nanoparticle catalyst, and the tin oxide nanotubehaving mesopores and macropores. As illustrated in FIG. 17, it can beseen that the region of the exhaled breath of the simulated diabeticpatients is apparently distinguished from the region of the exhaledbreath of the 10 healthy people, and it is confirmed that it is possibleto diagnose diabetes through the exhaled breath by using the materialdeveloped in the inventive concept.

In Experimental Example 1, the sensing characteristics of the gassensing material with respect to the biomarker gases are evaluated.However, it is expected that the gas sensing material exhibits excellentsensing characteristics with respect to H₂, NO_(x), SO_(x), HCHO, CO₂ ofharmful environmental gases as well, and a gas sensor exhibitingexcellent selectivity to harmful gases other than acetone can bemanufactured by changing the catalyst from the Pt nanoparticle catalystembedded in an apoferritin to various kinds of catalytic particles whichare synthesized by embedding catalytic materials such as Au, Pd, Rh, Cr,Co, and Ni in an apoferritin. In addition, it is possible to manufacturea nanosensor array exhibiting ultrahigh sensitivity and high selectivityby using a one-dimensional porous multi-kind metal oxide nanotube havinga double pore distribution that is synthesized using various kinds ofmetal oxides to serve as a sensing material matrix and thus themulti-kind catalytic particles have a number of circular or ellipticalpores. The one-dimensional porous metal oxide nanotube sensing materialwhich has a double pore distribution and is loaded with a nanoparticlecatalyst obtained from an apoferritin template can be used in anexcellent gas sensor for detection of harmful environmental gases and agas sensor for healthcare for volatile organic compound gas analysis inthe exhaled breath and diagnosis of a disease.

Hereinafter, a gas sensor member and a gas sensor which use a metaloxide semiconductor nanotube structure containing a nanoparticlecatalyst synthesized using an apoferritin, and manufacturing methodthereof will be described in detail with reference to the accompanyingdrawings.

In the inventive concept, a nanotube structure is synthesized in whichnanoparticle catalysts are uniformly distributed on the surface and inthe inside of the nanotube shell by controlling the heating rate duringthe heat treatment of a tin oxide precursor/polymer composite nanofibercontaining a nanoparticle catalyst synthesized using an apoferritin. Inorder to improve the sensing characteristics of the gas sensor using ametal oxide of the prior art, studies for improving the sensingcharacteristics by increasing the surface area and the porosity so as toreact with a more amount of gas have been carried out, and also studiesfor promoting the catalytic reaction by loading a metal or metal oxidecatalyst to the sensing material have been carried out. However, suchstudies have a disadvantage that the step of forming pores and the stepof loading the catalyst on the nanofiber are separately required. Inparticular, there is a disadvantage that the step of synthesizing ametal or metal oxide nanoparticle catalyst and uniformly loading it onthe nanofiber and the step of synthesizing nanoparticle catalyst havinga size of several nm are fairly complicated, and there is a disadvantagethat the step of synthesizing a tube structure or forming pores in orderto increase the specific surface area is also relatively complicated,takes a long time, and requires a high cost. In order to overcome such adisadvantage, in the inventive concept, a nanoparticle catalyst having asize of from 0.1 nm to 8 nm is synthesized using an apoferritin, this ismixed with a metal oxide precursor/polymer mixed electrospinningsolution, and then the nanoparticle catalyst is uniformly loaded on thesurface and in the inside of the metal oxide precursor/polymer compositenanofiber by conducting the electrospinning. Moreover, the proteintemplate encapsulating the nanoparticle catalyst and the polymer removedat the same time during a high-temperature heat treatment by controllingthe heating rate, a metal oxide nanotube structure containing thenanoparticle catalyst through the Ostwald ripening, and thus it ispossible to easily synthesize a sensing material in which nanoparticlecatalysts are uniformly loaded on a nanotube structure having a largespecific surface area without aggregation in a large scale by a singleprocess. Here, the metal oxide semiconductor nanotube in which thenanoparticle catalysts are uniformly distributed on the outside and inthe inside of the nanotube can maximize the catalytic effect when a gasreacts with the sensing material since the catalyst is uniformlydistributed, the nanotube structure formed by controlling the heatingrate during the heat treatment facilitates the penetration of the gasinto the tube, an effective surface reaction of the gas is induced dueto the increased surface area, and thus it is possible to manufacture ahighly sensitive gas sensing material. In particular, it is possible tosynthesize a variety of metal or metal oxide nanoparticles in the insideof an apoferritin protein and to fabricate a gas sensor exhibitingselectivity to a specific gas. In order to fabricate a gas sensor memberhaving the characteristics described above, a gas sensor member, a gassensor, and manufacturing methods thereof are implemented by anefficient and easy process.

FIG. 18 is a schematic diagram of a gas sensor member 1800 using a metaloxide semiconductor nanotube structure 1810 containing a nanoparticlecatalyst 1821 according to an embodiment of the inventive concept. Bysubjecting a composite nanofiber fabricated by electrospinning anapoferritin protein encapsulating a nanoparticle catalyst in the hollowstructure thereof with a metal oxide precursor/polymer mixedelectrospinning solution to a high-temperature heat treatment at a highheating rate, it is possible to form a structure in which the tin oxideparticles gather on the surface to form a nanotube 1810 having an emptystructure and the nanoparticle catalysts 1821 are uniformly loaded inthe inside and on the outside of the tube structure.

Here, the metal that can be synthesized in the hollow structure of theapoferritin is not particularly limited as long as it is present in anionic form. Specific examples thereof may include Copper(II) nitrate,Copper(II) chloride, Cobalt(II) nitrate, Cobalt(II) acetate,Lanthanum(III) nitrate, Lanthanum(III) acetate, platinum(IV) chloride,platinum(II) acetate, gold(I, III) chloride, gold(III) acetate, silverchloride, silver acetate, Iron(III) chloride, Iron(III) acetate,Nickel(II) chloride, Nickel(II) acetate, Ruthenium(III) chloride,Ruthenium Acetate, Iridium(III) chloride, iridium acetate, Tantalum(V)chloride, and Palladium(II) chloride, and it is possible to synthesize ananoparticle catalyst of Pt, Pd, Rh, Ru, Ni, Co, Cr, Ir, Au, Ag, Zn, W,Sn, Sr, In, Pb, Fe, Cu, V, Ta, Sb, Sc, Ti, Mn, Ga, or Ge using theseprecursors. In this manner, the size of the nanoparticle catalyst can becontrolled by controlling the amount of precursor in a size range of 0.1nm to 8 nm by using an apoferritin as a template, and there is a greatadvantage that the nanoparticle catalysts are surrounded with anapoferritin protein shell having a hollow structure so as to befavorably dispersed even in the electrospinning solution without beingaggregated with one another. From the viewpoint of the role ofnanoparticle catalyst acting in the gas sensing material, there are ananoparticle catalyst of a noble metal, such as platinum (Pt) or gold(Au), which exhibits a chemical sensitization effect that theconcentration of adsorbed oxygen ions which involve in the surfacereaction is increased by promoting the decomposition reaction of oxygenmolecule in between the surface of the metal oxide and the air layer anda nanoparticle catalyst which exhibits an electronic sensitizationeffect that a catalytic reaction is caused by the oxidation, such asPdO, Co₃O₄, NiO, Cr₂O₃, CuO, Fe₂O₃, Fe₃O₄, TiO₂, ZnO, SnO₂, V₂O₅, orV₂O₃, which affects the improvement in sensing characteristics.

It is possible to favorably disperse the nanoparticle catalysts withoutaggregation when the nanoparticle catalysts 1821 synthesized using anapoferritin are loaded in the inside and on the outside of the nanotubestructure as compared to the nanoparticle catalysts synthesized using ageneral polyol process since a nanoparticle catalyst surrounded by aprotein shell is used. In this respect, it is possible to uniformly loadthe nanoparticle catalysts in the inside and on the outside of the metaloxide precursor/polymer nanofiber as the nanoparticle catalysts areadded to the metal oxide precursor/polymer mixed electrospinningsolution and the electrospinning is then conducted. Here, the nucleationand particle growth occur through the high-temperature heat treatment ata heating rate of 10° C./min, and a metal oxide nanotube structurecontaining a nanoparticle catalyst can be formed through the Ostwaldripening. The diameter of the metal oxide nanotube structure containinga nanoparticle catalyst is in a diameter range of from 50 nm to 5 μm,the thickness between the inner and outer walls is in a range of from 10nm to 50 nm, and the length is in a range of from 1 μm to 100 μm.

The metal oxide semiconductor nanotube constituting the nanostructure isnot particularly limited to a specific material as long as the value ofthe electrical resistance and the electrical conductivity is changed bythe adsorption and desorption of gas. Specifically, the nanotube may bea nanotube composed of one or a composite material of two or moreselected from ZnO, SnO₂, WO₃, Fe₂O₃, Fe₃O₄, NiO, TiO₂, CuO, In₂O₃,Zn₂SnO₄, Co₃O₄, PdO, LaCoO₃, NiCo₂O₄, Ca₂Mn₃O₈, V₂O₅, Cr₂O₃, Nd₂O₃,Sm₂O₃, Eu₂O₃, Gd₂O₃, Tb₄O₇, Dy₂O₃, Ho₂O₃, Er₂O₃, Yb₂O₃, Lu₂O₃, Ag₂V₄O₁₁,Ag₂O, Li_(0.3)La_(0.57)TiO₃, LiV₃O₈, InTaO₄, CaCu₃Ti₄O₁₂, Ag₃PO₄,BaTiO₃, NiTiO₃, SrTiO₃, Sr₂Nb₂O₇, Sr₂Ta₂O₇, orBa_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O₃₋₇.

By using the gas sensor member 1800 using the metal oxide semiconductornanotube 1810 containing the nanoparticle catalyst 1821 described above,it is possible to configure a ultrahigh sensitive sensor which candiagnose a human body disease at the early stage by selectively sensinga specific gas which acts as a biomarker in the exhaled breath of ahuman body and can also be applied to an environmental sensor capable ofmonitoring harmful environmental gases. In addition, it is possible toquantitatively control the loading amount of the nanoparticle catalystthat is loaded on the nanotube while quantitatively controlling theamount of the nanoparticle catalyst and thus the catalytic propertiescan be effectively controlled, it is possible to effectively control thesurface area of the fibers from a structure in which the inside of thenanofiber is filled to a nanotube structure by controlling the heatingrate during the heat treatment, and thus there is also an advantage ofbeing able to rapidly and easily fabricate a variety of gas sensormembers.

FIG. 19 is a flow chart of a manufacturing method of a gas sensor memberusing a metal oxide semiconductor nanotube containing a nanoparticlecatalyst synthesized using the electrospinning method according to anembodiment of the inventive concept. As illustrated in the flow chart ofFIG. 19, the manufacturing method of a gas sensor member may include astep S1910 of synthesizing a nanoparticle catalyst using an apoferritin,a step S1920 of preparing a composite electrospinning solution by addingthe nanoparticle catalyst synthesized in the previous step to a metaloxide precursor/polymer electrospinning solution, a step S1930 ofsynthesizing a metal oxide precursor/polymer composite nanofibercontaining the nanoparticle catalyst synthesized using an apoferritinfrom the composite electrospinning solution utilizing electrospinningequipment, and a step S1940 of synthesizing a metal oxide nanotubeloaded with the nanoparticle catalyst through a high-temperature heattreatment at a relatively high heating rate of 10° C./min. Therespective steps will be described below in more detail.

First, the step S1910 of synthesizing a nanoparticle catalyst using anapoferritin is described.

The apoferritin used in this step S1910 includes ferritin extracted fromequine spleen, and an apoferritin prepared by removing the iron ionpresent inside a ferritin obtained from liver, spleen, or the like of ahuman body or a swine regardless of the extraction site may be used. Asthe method for removing the iron from the inside of the ferritin havinga structure surrounded with a protein, it is possible to use either of achemical method or an electrical method. Solutions of sodium chloride(NaCl) at various concentrations including saline can be used as asolution to keep the apoferritin having a hollow structure having anempty space therein, and the apoferritin is required to be stored underrefrigeration at 4° C. or lower. In addition, in order to embed a metalsalt in the apoferritin, a basic solution having a pH in a range of from8.0 to 9.5 is preferable, the apoferritin is immersed in a solutioncontaining a metal salt dissolved therein for about from 1 hour to 24hour so that the metal salt can be sufficiently diffused into theapoferritin. The concentration of the solution for storage, such assaline containing the apoferritin is set to be in a range of from 0.1 to200 mg/ml. As the solvent used when preparing the metal salt solution, acommercially available solvent such as ethanol, water, chloroform,N,N′-dimethylformamide, dimethylsulfoxide, N,N′-dimethylacetamide, orN-methylpyrrolidone can be used, and the solvent is not limited to aparticular solvent as long as it dissolves a metal salt. The kind andform of the metal salt synthesized in the inside of the apoferritin isnot particularly limited as long it is a precursor in an ionic form. Themetal salt is preferably a precursor in a salt form which can embed Pt,Pd, Rh, Ru, Ni, Co, Cr, Ir, Au, Ag, Zn, W, Sn, Sr, In, Pb, Fe, Cu, V,Ta, Sb, Sc, Ti, Mn, Ga, or Ge in the apoferritin, and through thehigh-temperature heat treatment, the protein is removed and thenanoparticle catalysts are converted to a metal or metal oxide catalyticparticles. At this time, the metal particles which are prone to beoxidized are easily converted to metal oxide particles. Such metal oxideparticles may exhibit the properties of n-type or p-type semiconductor.As the reducing agent to reduce the metal salt contained inside thehollow structure of the apoferritin, a generally used reducing agentsuch as sodium borohydride (NaBH₄), formic acid (HCOOH), oxalic acid(C₂H₂O₄) or lithium aluminum hydride (LiAIH₄) may be used, and areducing agent capable of reducing the metal salt so as to form ametallic nanoparticle catalyst may be used without any particularlimitation. The solution subjected to the reduction of the metal saltinside the apoferritin by a reducing agent is then subjected to thecentrifugation to separate the apoferritin protein embedding thenanoparticle catalyst therefrom, and the rotational speed of thecentrifugal separator used at this time is preferably from 10,000 rpm to13,000 rpm.

Subsequently, the step S1920 of preparing a metal oxideprecursor/polymer mixed electrospinning solution containing the metallicnanoparticle catalyst synthesized using an apoferritin above isdescribed.

In this step S1920, the apoferritin protein embedding the nanoparticlecatalyst synthesized in the previous step is added to a metal oxideprecursor/polymer mixed electrospinning solution to prepare a mixedelectrospinning solution in which the nanoparticle catalysts areuniformly dispersed in the electrospinning solution. Here, as thesolvent, a commercially available solvent such asN,N′-dimethylformamide, dimethylsulfoxide, N,N′-dimethylacetamide,N-methylpyrrolidone, deionized (DI) water, or ethanol may be used, butit is required to select a solvent which can dissolve the metal oxideprecursor and the polymer at the same time. In addition, the polymerused herein is not limited to a specific polymer as long as it is apolymer that is removed through the high-temperature heat treatment.Specific examples of the polymer that can be used in this step S1920 mayinclude polymethyl methacrylate (PMMA), polyvinylpyrrolidone (PVP),polyvinyl acetate (PVAc), polyvinyl alcohol (PVA), polyacrylonitrile(PAN), polyethylene oxide (PEO), polypropylene oxide (PPO), polyethyleneoxide copolymer, polypropylene oxide copolymer, polycarbonate (PC),polyvinylchloride (PVC), polycaprolactone, and polyvinylidene fluoride.

The metal oxide precursor used in this step is not limited to a specificmetal salt as long as it is a precursor which dissolves in a solvent andcontains a metal salt capable of forming a metal oxide semiconductornanofiber or nanotube exhibiting gas sensing characteristics through thehigh-temperature heat treatment, such as SnO₂, WO₃, CuO, NiO, ZnO,Zn₂SnO₄, Co₃O₄, Cr₂O₃, LaCoO₃, V₂O₅, IrO₂, TiO₂, Er₂O₃, Tb₂O₃, Lu₂O₃,Ag₂O, SrTiO₃, Sr₂Ta₂O₇, BaTiO₃, or Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O₃₋₇.

The ratio of the metal oxide precursor to the polymer for forming theelectrospinning solution is preferably about from 1:0.5 to 2, and theratio to the polymer to the nanoparticle catalyst synthesized using theapoferritin is preferably about from 1:0.00001 to 1:0.1. The kind of themetal salt encapsulated in the apoferritin is preferably selected inconsideration of the sensing characteristics and selectivity of the gasto be sensed, and it is possible to manufacture a gas sensor memberexhibiting various characteristics by changing the metal salt.

As the procedure to prepare an electrospinning solution in the stepS1920, the metal oxide precursor is first dissolved in a solvent, asolution of the apoferritin encapsulating the nanoparticle catalystsynthesized in advance is added thereto, and the mixed solution is mixedso as to uniformly disperse the apoferritin encapsulating thenanoparticle catalyst. After the mixed solution is sufficiently mixed, apolymer is added thereto at an appropriate ratio, and the mixture isstirred until the polymer is completely dissolved in the solution. Asthe stirring condition, the mixture is preferably stirred at from roomtemperature to 50° C., and it is sufficiently stirred for from 5 hoursto about 48 hours so that the apoferritin encapsulating the nanoparticlecatalyst, the metal oxide precursor, and the polymer are uniformly mixedin the solution. The electrospinning solution synthesized above iselectrospun, and the step 1930 of fabricating the metal oxideprecursor/polymer composite nanofiber containing the apoferritin proteinencapsulating the nanoparticle catalyst through the electrospinning.

Upon carrying out the electrospinning method in order to carry out thestep S1930, the electrospinning solution containing the metal oxideprecursor/polymer and the apoferritin protein encapsulating thenanoparticle catalyst thus prepared is filled in a syringe, the syringeis pressed using the syringe pump at a constant rate so that a certainamount of the electrospinning solution is discharged therefrom. Theelectrospinning system may be constituted by a high voltage apparatus, agrounded conductive substrate, a syringe, and a syringe nozzle, a highvoltage about from 5 kV to 30 kV is applied to between the solutionfilled in the syringe and the conductive substrate to generate theelectric field, and the electrospinning solution discharged through thesyringe nozzle is ejected in a long nanofiber form due to the electricfield thus generated, whereby the electrospinning is conducted. Theelectrospinning solution ejected in a long nanofiber form is obtained asa solid polymer fiber as the solvent contained in the electrospinningsolution is evaporated and volatilized and a composite fiber containingthe metal oxide precursor and the apoferritin encapsulating thenanoparticle catalyst is fabricated at the same time. The dischargespeed may be controlled to about from 0.01 ml/min to 0.5 ml/min, and itis possible to fabricate a metal oxide precursor/polymer/nanoparticlecatalyst composite nanofiber having a desired diameter by controllingthe voltage and the discharge rate.

Finally, in the step S1940 of fabricating a metal oxide nanotubestructure in which the nanoparticle catalysts are uniformly distributedwithout being aggregated with one another through the high-temperatureheat treatment of the composite nanofiber fabricated in the previousstep, a metal oxide nanotube structure can be formed by controlling theheating rate during the heat treatment. The polymer and the proteinencapsulating the nanoparticle catalyst are all decomposed and removedthrough the heat treatment at a temperature of 400 to 800° C., the metaloxide precursor and nanoparticle catalyst in the inside of the nanofiberdiffuse toward the nanofiber surface through the Ostwald ripening, and ametal oxide nanotube structure is finally formed. It is possible tofabricate the metal oxide nanotube structure 1810 in which thenanoparticle catalysts are densely dispersed in the shell structure ofthe metal oxide nanotube by setting the heating rate to be relativelyhigh of 10° C./min during this process.

FIG. 20 is a diagram illustrating a manufacturing process according tothe method for manufacturing a gas sensor member using a metal oxidesemiconductor nanotube containing a nanoparticle catalyst using anelectrospinning method according to an embodiment of the inventiveconcept.

The step S2010 of the first step illustrates an example of fabricating ananofiber from an electrospinning solution 2010 containing a metal oxideprecursor (tin precursor)/polymer and a nanoparticle catalyst embeddedin the apoferritin. The nanofiber 2030 that is fabricated through theprocess described above and illustrated in FIG. 20 has a feature inwhich the apoferritin 2020 encapsulating a nanoparticle catalyst isuniformly distributed.

The step S2020 of the second step illustrates the high-temperature heattreatment of the composite nanofiber synthesized in the step S2010. Theheat treatment is conducted while increasing the temperature up to 600°C. at a relatively high heating rate of 10° C./min so that the polymerand the protein encapsulating the nanoparticle catalyst are all removedand the metal oxide and the nanoparticle catalyst all diffuse toward thenanofiber surface, whereby a metal oxide semiconductor nanotube 2040uniformly containing a metallic nanoparticle catalyst is synthesized.

In this embodiment of FIG. 20, an example of manufacturing a tin oxidenanotube structure using a tin oxide precursor is described, but themetal oxide precursor is not particularly limited as long as it containsa metal salt as described above.

As described above, in the method for manufacturing the gas sensormember 1800 using the metal oxide semiconductor nanotube 1810 containingthe nanoparticle catalyst 1821 fabricated by using the electrospinningmethod and controlling the heating rate in the heat treatment accordingto embodiments of the inventive concept, a one-dimensional nanotubestructure having a large surface area for the reaction with a gas isformed and a catalyst that is uniformly dispersed and exhibits achemical/electronic sensitization effect is loaded thereto using theproperties of a protein, and thus it is possible to improve the responserate characteristics, sensitivity characteristics, and selectivity ofthe gas sensor.

Hereinafter, the inventive concept will be described in detail withreference to Embodiment Examples and Comparative Examples. EmbodimentExamples and Comparative Examples are only for explaining the inventiveconcept, and the inventive concept is not limited to the followingEmbodiment Examples.

Embodiment Example 3: Preparation of Pt and Au Nanoparticle CatalystUsing Apoferritin as Template

The following procedure is used in order to synthesize Pt and Aunanoparticle catalysts inside an apoferritin having a hollow structure.

The pH of 1 ml of an apoferritin solution (Sigma Aldrich) dispersed in a0.15 M NaCl aqueous solution at a concentration of 35 mg/ml is adjustedto 8.6 using NaOH, thereby preparing a condition for the diffusion of ametal salt diffuse into the apoferritin. The basic solution used hereinis not particularly limited as long as it is a basic solution. Next,H₂PtCl₆.H₂O and H₂AuCl₆.H₂O are used as the Pt precursor and Auprecursor which are required for the synthesis of the Pt and Aunanoparticle catalysts. In DI water, 16 mg of H₂PtCl₆.H₂O and 16 mg ofH₂AuCl₆.H₂O are respectively dissolved to prepare aqueous solutions. Thetwo aqueous solutions of metal salt precursors thus prepared aregradually dropped into the apoferritin solution having an adjusted pHdrop and stirred so that the Pt and Au slats diffuse into the hollow ofthe apoferritin and are embedded therein. The stirring conditionsreferred to herein means that the stirring is conducted at roomtemperature and a rotational speed of 100 rpm for about 1 hour. Afterthe metal salts are embedded in the apoferritin, the metal ions(Pt⁴⁺/Au⁴⁺) in the hollow of the apoferritin are reduced to metals(Pt/Au) using a reducing agent so as to form a nanoparticle catalyst.The reducing agent NaBH₄ used at this time is prepared as an aqueoussolution at a concentration of 40 mM and added by 0.5 ml.

The aqueous solution in which the Pt nanoparticle catalyst and the Aunanoparticle catalyst which are synthesized using the apoferritin by themethod as described above are dispersed contains the reducing agent andthe ligand of the metal salts in a great amount, and thus thesynthesized metallic nanoparticle catalysts are required to be extractedusing a centrifugal separator. As the centrifugal condition, it ispreferable to conduct the centrifugation at about from 10,000 rpm to12,000 rpm for 10 minutes or longer. The apoferritin encapsulating thePt nanoparticle catalyst and Au nanoparticle catalyst thus extracted bycentrifugation is dispersed in water again, thereby preparing an aqueoussolution in which the Pt nanoparticle catalyst and the Au nanoparticlecatalyst are dispersed in the inside of the apoferritin.

FIG. 24 is TEM images of the apoferritin encapsulating the Ptnanoparticle catalyst and Au nanoparticle catalyst prepared by the aboveprocess. It can be seen that the apoferritin encapsulating the Ptnanoparticle catalyst and Au nanoparticle catalyst thus synthesized hasa diameter of about from 2 to 5 nm and a spherical shape.

Embodiment Example 4: Fabrication of tin oxide (SnO₂) nanotube 2040structure containing Pt and Au nanoparticle catalysts

First, 0.25 g of tin chloride dihydrate of the metal oxide precursor isadded to a mixed solvent of 1.35 g of DMF and 1.35 g of ethanol anddissolved at room temperature. Next, 200 mg of the aqueous solution ofthe apoferritin 2020 encapsulating the Pt nanoparticle catalyst and Aunanoparticle catalyst prepared in Embodiment Example 3 is added to andmixed with two tine oxide precursor/mixed solvent electrospinningsolutions, respectively. In order to increase the viscosity of thesolutions in which the apoferritin particles encapsulating the Ptnanoparticle catalyst and Au nanoparticle catalyst and the tine oxideprecursor are uniformly mixed, 0.35 g of polyvinylpyrrolidone (PVP)polymer having a molecular weight of 1,300,000 g/mol is added thereto,respectively, and the mixtures are stirred at room temperature and arotational speed of 500 rpm for 24 hours to prepare electrospinningsolutions. The electrospinning solutions thus prepared are filled in thesyringe (Henke-Sass Wolf, 10 mL NORM-JECT®), the syringe is connected tothe syringe pump, the electrospinning solution is pushed out at adischarge speed of 0.1 ml/min, and voltage between the nozzle (needle,27 gauge) used during electrospinning and the collector gathering thenanofibers is set to 14 kV, thereby conducting the electrospinning. Atthis time, stainless steel plate is used as the nanofiber collectingplate, and the distance between the nozzle and the collector is set to15 cm.

FIG. 21 shows SEM images of the tin oxide precursor/polyvinylpyrrolidonecomposite nanofiber containing the Pt nanoparticle catalyst and the tinoxide precursor/polyvinylpyrrolidone composite nanofiber containing theAu nanoparticle catalyst obtained after the electrospinning. It can beseen that a one-dimensional nanofiber is synthesized, and the diameterhas a value in between 200 nm to 300 nm.

The metal oxide precursor/polymer composite nanofiber loaded with the Ptnanoparticle catalyst and the metal oxide precursor/polymer compositenanofiber loaded with the Au nanoparticle catalyst fabricated by themethod as described above are respectively maintained at 600° C. for 1hour at a heating rate of 10° C./min and then cooled to room temperatureat a cooling rate of 40° C./min. The heat treatment is conducted in anair atmosphere using the small electric furnace Vulcan 3-550manufactured by Ney. The apoferritin protein encapsulating thenanoparticle catalyst and the polymer are all decomposed through thehigh-temperature heat treatment. In addition, the heat treatment isconducted in the air atmosphere, thus the tin salt precursor on thenanofiber surface is first oxidized into tin oxide particles through thenucleation and particle growth, the tin salt precursor inside thenanofiber is also oxidized through the Ostwald ripening and diffusedtoward the nanofiber surface to form a tin oxide nanotube, and the Ptnanoparticle catalyst and Au nanoparticle catalyst contained in thenanofiber surface also diffuse toward the nanotube surface to form a tinoxide nanotube uniformly loaded with the Pt nanoparticle catalyst and atin oxide nanotube uniformly loaded with the Au nanoparticle catalyst.

FIG. 25 shows SEM images of the tin oxide nanotube loaded with the Ptnanoparticle catalyst and the tin oxide nanotube loaded with the Aunanoparticle catalyst obtained after the heat treatment according toEmbodiment Example 4. The outer diameter of the nanotube structure thusformed has a size of about from 50 nm to 2 μm and the inner diameterthereof has a size of about from 40 nm to 1.9 μm. The thickness of thetube is about from 10 to 100 nm.

FIG. 26 shows TEM images of the tin oxide nanotube containing the Ptnanoparticle catalyst fabricated in Embodiment Example 4. It can be seenthat the Pt nanoparticle catalysts are present in the tin oxide nanotubethrough the transmission electron microscope grid analysis and the Ptnanoparticle catalysts a crystallized in the tin oxide nanotube throughselected area electron diffraction (SAED) pattern. In addition, it canbe seen that the Pt nanoparticle catalysts are uniformly distributed inthe formed tin oxide nanotube structure thus through the componentanalysis (EDS) image by the TEM analysis.

FIG. 27 shows TEM images of the tin oxide nanotube containing the Aunanoparticle catalyst synthesized in Embodiment Example 4. It can beseen that the Au nanoparticle catalysts are present in the tin oxidenanotube through the transmission electron microscope grid analysis andthe Au nanoparticle catalysts a crystallized in the tin oxide nanotubethrough selected area electron diffraction (SAED) pattern. In addition,it can be seen that the Au nanoparticle catalysts are uniformlydistributed in the formed tin oxide nanotube structure through thecomponent analysis (EDS) images by the TEM analysis.

Comparative Example 3: Fabrication of Pure Tin Oxide Nanofiber NotContaining Nanoparticle Catalyst

As Comparative Example 3 to be compared with Embodiment Example 3, apure tin oxide nanofiber is fabricated by not adding a nanoparticlecatalyst embedded in an apoferritin. Specifically, 0.35 g ofpolyvinylpyrrolidone (PVP) polymer having an average weight of 1,300,000g/mol and 0.25 g of tin chloride dihydrate of the tin oxide precursorare dissolved in a mixed solvent composed of 1.35 g of DMF and 1.35 g ofethanol at room temperature for about 24 hours under a condition of 500rpm. After the stirring is completed, the tin oxide precursor/polymermixed electrospinning solution is filled in the syringe forelectrospinning (Henke-Sass Wolf, 10 mL NORM-JECT®), the syringe isconnected to the syringe pump, the electrospinning solution is pushedout at a discharge speed of 0.1 ml/min, the needle used duringelectrospinning is a 27 gauge needle, and a high voltage of about 14 kVis applied while maintaining the distance between the nozzle thecollector to collect the nanofibers to be 15 cm, thereby fabricating thetin oxide precursor/polymer composite nanofiber web. The tin oxideprecursor/polymer composite nanofiber fabricated above is subjected to ahigh-temperature heat treatment to remove the polymer, and the tin oxideprecursor is converted to tin oxide by oxidation. The high-temperatureheat treatment is conducted for 1 hour at 600° C., the heating rate isconstantly maintained at 4° C./min, and the cooling rate is constantlymaintained at 40° C./min.

FIG. 22 is a SEM image of the pure tin oxide nanofiber fabricated not byadding a nanoparticle catalyst in Comparative Example 3. It can be seenthat the tin oxide nanofiber thus fabricated has a diameter of aboutfrom 50 nm to 2 μm and the nanofiber structure has a cylindricalstructure.

The pure tin oxide nanofiber fabricated above is was used for comparisonon the sensing characteristics with respect to various kinds of gasestogether with the tin oxide nanotube loaded with the Pt nanoparticlecatalyst and tin oxide nanotube loaded with the Au nanoparticle catalystfabricated in Embodiment Example 4.

Comparative Example 4: Fabrication of Pure Tin Oxide Nanotube NotContaining Nanoparticle Catalyst

As Comparative Example 4 to be compared with Embodiment Example 4, apure tin oxide nanotube is fabricated by not adding Pt and Aunanoparticle catalysts embedded in an apoferritin. Specifically, 0.35 gof polyvinylpyrrolidone (PVP) polymer having an average weight of1,300,000 g/mol and 0.25 g of tin chloride dihydrate of the tin oxideprecursor are dissolved in a mixed solvent composed of 1.35 g of DMF and1.35 g of ethanol at room temperature for about 24 hours under acondition of 500 rpm. After the stirring is completed, the tin oxideprecursor/polymer mixed electrospinning solution is filled in thesyringe for electrospinning (Henke-Sass Wolf, 10 mL NORM-JECT®), thesyringe is connected to the syringe pump, the electrospinning solutionis pushed out at a discharge speed of 0.1 ml/min, the needle used duringelectrospinning is a 27 gauge needle, and a high voltage of about 14 kVis applied while maintaining the distance between the nozzle thecollector to collect the nanofibers to be 15 cm, thereby fabricating thetin oxide precursor/polymer composite nanofiber.

The tin oxide precursor/polymer composite nanofiber thus synthesized issubjected to a high-temperature heat treatment to remove the polymer,and the tin oxide precursor is converted to tin oxide by oxidation. Thehigh-temperature heat treatment is conducted for 1 hour at 600° C., theheating rate is constantly maintained at 10° C./min, and the coolingrate is constantly maintained at 40° C./min. Here, the heating rate isset to 10° C./min in order to increase the temperature at a higher speedthan in Comparative Example 3 in which the heating rate is set to 4°C./min, and this is for the formation of the nanotube structure.

FIG. 23 is a SEM image of the tin oxide nanotube fabricated inComparative Example 4. The outer diameter of the pure tin oxide nanotubethus fabricated has a size of about from 50 nm to 2 μm and the innerdiameter thereof has a size of about from 40 nm to 1.9 μm. The thicknessof the tube is about from 10 to 100 nm.

Experimental Example 2: Manufacture of gas sensor using tin oxidenanotube loaded with platinum (Pt) nanoparticle catalyst, tin oxidenanotube loaded with gold (Au) nanoparticle catalyst, pure tin oxidenanotube, and pure tin oxide nanofiber, and evaluation oncharacteristics thereof.

In order to manufacture exhaled breath sensors using the sensingmaterials for gas sensor fabricated in Embodiment Examples 3 and 4 andComparative Examples 3 and 4, 5 mg of each of the tin oxide nanotubeloaded with the platinum (Pt) nanoparticle catalyst that is partiallyoxidized through the high-temperature heat treatment, the tin oxidenanotube loaded with the gold (Au) nanoparticle catalyst that ispartially oxidized through the high-temperature heat treatment, the puretin oxide nanotube, and the pure tin oxide nanofiber is dispersed in 100μl of ethanol and subjected to the ultrasonic cleaning for 1 hour to beground. During the grinding, the nanotube structure synthesized abovemay have a nanotube structure shortened in the longitudinal direction ora nano-rod structure.

The tin oxide nanotube 1810 loaded with the Pt nanoparticle catalyst orthe Au nanoparticle catalyst, the pure tin oxide nanofiber, and the puretin oxide nanotube are coated on the upper portion of the aluminasubstrate which has a size of 3 mm×3 mm and on which parallel gold (Au)electrodes are formed at an interval of 300 μm by drop coating. As thecoating process, 2 μl of a mixed solution of the tin oxide nanotubeloaded with the Pt nanoparticle catalyst, the tin oxide nanotube loadedwith the Au nanoparticle catalyst, the pure tin oxide nanotube, and thepure tin oxide nanofiber dispersed in ethanol was respectively coated onthe alumina substrate having the sensor electrode using a micropipetteand dried on a hot plate at 60° C., and this process was repeated 4 to 6times so as to coat a sufficient amount of the tin oxide nanotube loadedwith the Pt nanoparticle catalyst, the tin oxide nanotube loaded withthe Au nanoparticle catalyst, the pure tin oxide nanotube, and the puretin oxide nanofiber on the upper portion of the alumina sensing plate.

In addition, the response characteristics of the gas sensor thusfabricated with respect to acetone (CH₃COCH₃), hydrogen sulfide (H₂S),and toluene (C₆H₅CH₃) of the biomarker gas for the diagnosis ofdiabetes, halitosis, and lung cancer, respectively, were evaluated at arelative humidity of 85-95% RH to be similar to the humidity of gases inthe exhaled breath of a human body by changing the concentration of therespective gases from 5 to 4, 3, 2, and 1 ppm and maintaining thedriving temperature of the sensor at 350° C. at the same time. Inaddition, in Experimental Example 2, the selective gas sensingcharacteristics were investigated by evaluating the sensingcharacteristics not only for acetone (CH₃COCH₃), hydrogen sulfide (H₂S),and toluene (C₆H₅CH₃) of the representative examples of the volatileorganic compound gas but also for nitric oxide (NO), carbon monoxide(CO), ammonia (NH₃), and pentane (C₅H₁₂) of the biomarker gas of asthma,chronic obstructive pulmonary disease, nephritis, and heart disease.

FIG. 28 is a graph illustrating the time course response properties(R_(air)/R_(gas), where R_(air) means the resistance value of the metaloxide material when the air is injected, and R_(gas) means theresistance value of the metal oxide material when acetone is injected)when the concentration of acetone decreases from 5 to 4, 3, 2, and 1 ppmat 350° C.

As illustrated in FIG. 28, the sensing characteristics of the sensormanufactured using the tin oxide nanotube 1810 on which the Ptnanoparticle catalyst embedded in the apoferritin is loaded through theheat treatment with respect to acetone is 8.27 times as high as that ofthe pure tin oxide nanotube and 18.95 times as high as that of the puretin oxide nanofiber.

FIG. 29 is a graph illustrating the sensor test results showing the timecourse response properties when the concentration of hydrogen sulfidedecreases from 5 to 4, 3, 2, and 1 ppm at 350° C.

As illustrated in FIG. 29, the sensing characteristics of the sensormanufactured using the tin oxide nanotube 1810 on which the Ptnanoparticle catalyst embedded in the apoferritin is loaded through theheat treatment with respect to hydrogen sulfide is 4.23 times as high asthat of the pure tin oxide nanotube and 11.03 times as high as that ofthe pure tin oxide nanofiber.

FIG. 30 is a graph illustrating the sensor test results showing the timecourse response properties when the concentration of toluene decreasesfrom 5 to 4, 3, 2, and 1 ppm at 350° C.

As illustrated in FIG. 30, the sensing characteristics of the sensormanufactured using the tin oxide nanotube 1810 on which the Ptnanoparticle catalyst embedded in the apoferritin is loaded through theheat treatment with respect to toluene is 1.12 times as high as that ofthe pure tin oxide nanotube and 1.76 times as high as that of the puretin oxide nanofiber.

FIG. 31 is a graph illustrating the response value of the sensormanufactured using the tin oxide nanotube on which the Pt nanoparticlecatalyst embedded in the apoferritin is loaded through the heattreatment with respect to hydrogen sulfide, toluene, nitrogen monoxide,carbon monoxide, ammonia, and pentane of biomarker gases of otherdiseases together with the response value with respect to acetone of thebiomarker gas of diabetes and lipolysis at a concentration of 1 ppm and350° C.

As illustrated in FIG. 31, the gas sensor manufactured using the tinoxide nanotube 1810 on which the Pt nanoparticle catalyst embedded inthe apoferritin is loaded through the heat treatment exhibitsspecifically excellent selective sensing characteristics with respect toacetone of the biomarker gas of diabetes and lipolysis compared tohydrogen sulfide, toluene, pentane, carbon monoxide, ammonia, andnitrogen monoxide of biomarker gases of other diseases.

FIG. 32 is a graph illustrating the hydrogen sulfide (1 to 5 ppm)response properties of the sensor manufactured using the tin oxidenanotube which is fabricated in Embodiment Example 4 on which the gold(Au) nanoparticle catalyst embedded in the apoferritin is loaded throughthe heat treatment.

FIG. 33 is a graph illustrating the response value of the sensormanufactured using the tin oxide nanotube on which the gold (Au)nanoparticle catalyst embedded in the apoferritin is loaded through theheat treatment with respect to toluene, acetone, ammonia, and ethanol ofbiomarker gases of other diseases together with the response value withrespect to hydrogen sulfide of the biomarker gas of diabetes andlipolysis at a concentration of 1 ppm and 300° C.

In Experimental Example 2, the sensing characteristics of the gassensing material with respect to the volatile organic compound gases areevaluated. However, it is expected that the gas sensing materialexhibits excellent sensing characteristics with respect to H₂, NO_(x),SO_(x), HCHO, CO₂ of harmful environmental gases as well, and it hasbeen confirmed that a gas sensor exhibiting excellent selectivity toacetone and hydrogen sulfide is manufactured by changing the kind of thecatalyst in the sensor manufactured using the tin oxide nanotube onwhich the platinum (Pt) or gold (Au) nanoparticle catalyst embedded inthe apoferritin is loaded through the heat treatment. In addition, it ispossible to manufacture a nanosensor array exhibiting ultrahighsensitivity and high selectivity by using a multi-kind metal oxidenanotube loaded with various kinds of catalyst particles sinceadditional change in selectivity is expected by changing the kind ofmetal oxide materials. The metal oxide nanotube sensing material loadedwith a nanoparticle catalyst obtained from an apoferritin template canbe used in an excellent gas sensor for detection of harmfulenvironmental gases and a gas sensor for healthcare for volatile organiccompound gas analysis in the exhaled breath and diagnosis of a disease.

While the inventive concepts have been described with reference toexample embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirits and scopes of the inventive concepts. Therefore, itshould be understood that the above embodiments are not limiting, butillustrative. Thus, the scopes of the inventive concepts are to bedetermined by the broadest permissible interpretation of the followingclaims and their equivalents, and shall not be restricted or limited bythe foregoing description.

REFERENCE NUMERALS

100: one-dimensional porous metal oxide nanotube gas sensor member whichcontains nanoparticle catalyst and has double pore distributionincluding great number of circular or elliptical pores

110: nanoparticle catalyst in partially oxidized state after removal ofapoferritin shell through high-temperature heat treatment

121: circular or elliptical macropores formed as spherical polystyrenesacrificial template is decomposed through high-temperature heattreatment and macropores are filled by crystallization and diffusion ofmetal oxide

131: macropores formed as spherical polystyrene sacrificial template isdecomposed through high-temperature heat treatment

1800: metal oxide nanotube gas sensor member containing nanoparticlecatalyst

1810: metal oxide nanotube containing nanoparticle catalyst

1821: nanoparticle catalyst in partially oxidized state after removal ofapoferritin shell through high-temperature heat treatment

2010: tin oxide precursor/polymer electrospinning solution embeddingapoferritin containing Pt nanoparticle catalyst or Au nanoparticlecatalyst

2020: Pt nanoparticle catalyst synthesized using apoferritin

2030: tin oxide precursor/polymer nanofiber containing apoferritincontaining Pt nanoparticle catalyst or Au nanoparticle catalyst

2040: tin oxide nanotube containing partially oxidized Pt nanoparticlecatalyst or Au nanoparticle catalyst

2050: partially oxidized Pt nanoparticle catalyst or Au nanoparticlecatalyst after removal of apoferritin shell through high-temperatureheat treatment

What is claimed is:
 1. A method for manufacturing a metal oxide nanotubecomposite sensing material loaded with a catalyst and having a doubleaverage surface pore distribution, the method comprising: (a)synthesizing a first dispersion solution in which metallic nanoparticlecatalysts embedded in inner hollow structures of apoferritins andencapsulated in a protein are dispersed uniformly; (b) preparing anelectrospinning solution by mixing the first dispersion solution with asecond dispersion solution having a spherical polymer sacrificialtemplates dispersed therein and mixing the mixed dispersion solutionswith a solvent having a metal oxide precursor and a polymer dissolvedtherein; (c) forming a composite nanofiber having a plurality of thespherical polymer sacrificial templates and the metallic nanoparticlecatalysts embedded in the inner hollow structures of the apoferritinsand encapsulated in the protein distributed on the surface and in theinside of the metal oxide precursor/polymer composite nanofiber from theelectrospinning solution using an electrospinning method; and (d)forming a one-dimensional porous metal oxide nanotube having a circularor elliptical double surface pore distribution and having the metallicnanoparticle catalyst uniformly loaded in the inside and on the innersurface and outer surface of a shell constituting the nanotube asorganic substances including the polymer matrix constituting thenanofiber, the spherical polymer sacrificial template, and the proteinembedding the metallic nanoparticle catalyst are removed at the sametime during a heat treatment of the composite nanofiber.
 2. The methodaccording to claim 1, further comprising: (e) fabricating a resistancechange-type semiconductor gas sensor by dispersing or grinding theone-dimensional porous metal oxide nanotube and coating it on a sensorelectrode for a semiconductor type gas sensor using at least one coatingmethod among drop coating, spin coating, ink-jet printing, anddispensing, wherein a harmful environmental gas and a biomarker gas fordiagnosis of a disease can be detected using the resistance change-typesemiconductor gas sensor.
 3. The method according to claim 1, whereinthe step (a) is a process to produce the metallic nanoparticle catalystinside the apoferritin by substituting a metal salt into the apoferritinand reducing the substituted metal salt using a reducing agent, and thesolution containing the apoferritin has a pH in a range of from 2 to 3or from 7.5 to 9 and a salinity ratio in a range of from 0.1 mg/ml to150 mg/ml.
 4. The method according to claim 1, wherein the metallicnanoparticle catalysts are uniformly loaded on the outside and in theinside of the nanotube as the polymer is rapidly decomposed through theheat treatment having a heating rate of from 10° C./min to 50° C./min toincrease the amount of carbon dioxide and water vapor that is releasedper unit time utilizing a phenomenon that carbon dioxide and water vaporare generated when a polymer is decomposed through the heat treatment inthe step (d).